PSTN Switch Off? Colt supports ISDN services with 'Voice Line (v)'

ISDN over SIP - Technical Specifications

Home > Voice Services > PSTN Switch Off? Colt continues to support ISDN services

Discover more:
Login to Colt Online
Register for an account

Future proof your services

Alternative services

Additional services

PSTN Switch off ?

In contrast with most other operators and despite the global end of life of the traditional PSTN network, Colt continues to support ISDN services offering the ISDN over SIP product 'Voice Line (v)'.

Protect your telephony infrastructure investment!

PSTN networks are being switched off, but that does not mean the Public Telephony Network will fail us. Traditional telephony can go hand in hand with Voice over IP (VoIP) or Voice in the Cloud. All you need is a Colt managed Voice Gateway next to your PBX. This device will translate outgoing TDM traffic from your PBX to an IP consumable signal and vice versa.

You can postpone migrating to pure VoIP or the cloud and migrate only when you are ready to do so. Meanwhile, Colt can support you with your current PBX. No need to invest yet in a new IP PBX and buy IP Phones or headsets.... Once the Voice Gateway in place, you can take your time to choose the best option for your business and Colt can help you to transition your infrastructure to pure VoIP SIP Trunking or take your communication needs to the cloud. You don't have to do this all at once as hybrid solutions are possible: your can deploy UCaaS with Voice in the Cloud for your users while still make use of some vital traditional phones in your building.

At the same time, all operators can retire their huge space and power consuming switches.

Switching off the switches was part of Colt's End of Life program of the SDH network which was recently concluded. And guess what ? Many of our customers needed a bit more time to go all IP and are happy callers with our Voice Line (v) solution.

What is Colt's 'Voice Line (v)'?

Colt ensures that customers requiring ISDN services with a TDM PBX and phones remain supported, even if Colt carries the traffic over its future proof IP network. This ISDN over SIP service is called “Voice Line (v)”.

Service overview diagram

To allow Colt’s customers to retain a TDM PBX for ISDN telephony, Colt deploys voice gateways per site to allow the TDM signaling to be converted to IP (and vice versa). * Hybrid TDM-IP PBX’ are also supported

Depending upon the architecture, Colt may not install a physical router at the customer’s premises, implementing virtual routing instead. Alternatively, Colt offers a service installed over a non-Colt internet access.


  • Line format: ETSI
  • Protocol: ETS 300 102 / ITU Q.931 (ISDN30) – PRI (4/6/8 BRIs on request)
  • Codec: G.711
  • Line coding: hdb3
  • Framing: Double frame/Multiframe
  • Call type:  Speech
  • Speed: 64 kbit/s
  • Channels: 30 channel per PRI
  • Trunk selection sequence: Channel allocation from lowest available B channel - ‘Older’ B channel allocation from older available B channel
  • B + D glare: Yield
  • Amount of digit send/receive: As per RFC 4733
  • Clock pulse: The voice gateway generates the clock from its local oscillator (free running clock). 5PPM equates to 18ms clock slip per hour. By default the voice gateway uses the internal clock (free run use of internal oscillator without reference sources – each equipment uses its own internal clock, which can lead to minor differences). External clocking (Voice gateway is clock slave and syncs with the clock of the customer’s PBX) from a PRI can be enabled and is recommended when using modem passthrough and 64K data features which are sensitive to network clocking.


  • Enabled for White Labeling
  • Protocol: ETS 300 102 / ITU Q.931 (ISDN30), including VN4 (France), X11 (Italy)
  • Codec: G.711
  • Interfaces: 1 – 10 PRIs using the same ethernet connectivity
  • Connectivity, depending on availability: On-net: Colt fibre & Off-net: OLO fibre, wDSL, public internet
  • Fax: Group 3 T.38 Fax Relay of Fax – G.711 (Pass-through)
  • Modem: V.34 - 64kbps unrestricted data (excl. bonding)
  • Point of Sale: POS devices with modems operating with an answering tone of 2100 Hz only (V.25 modem tone)
  • CLI Presentation: Colt supports CLIP, CLIR, CLIP Screening and CLIR per call, Number Presentation, CLIP No Screening (depending on country regulation)
  • Hunt Groups: set according to call distribution (port group definition)
  • Loadshare and overflow routing
  • Dual Homing Voice Resiliency: Active/Standby or Active/Active
  • Multiple LACs (Local Area Code)
  • Encryption: In case the service is delivered over public internet instead of a Colt IP Access (standard bearer dedicated for voice traffic), the traffic is encrypted. Else, Colt is using static SIP peering (no SIP registration) to connect to the CPE devices. With this setup, adding encryption using mutual TLS means that each CPE device would require its own TLS certificate that needs to be administered/renewed. The security benefit of doing this does not justify the overhead, especially since the traffic is only transported over the COLT Internet Backbone (not public Internet).
  • QOS: For the standalone option (and the converged on-net option), the bandwidth is dedicated to voice traffic only, so QOS is not required on the access circuit, but we do use QoS across the IP backbone. For the converged offnet options, QOS is configured. COS in the ethernet circuit is not required as the access circuit is dedicated for Voice, hence the voice traffic is marked and is prioritised across the IP backbone.
  • Colt has a Voice Config Order enrichment portal which can be of importance during ordering process. It allows you to order the basic service while gathering the technical details regarding number format, CLI Presentation etc and provide these vie Colt Online, at a later stage. Only when all details are know, the configuration of the service will start.
  • Colt can both port in number ranges from other operators and offer new number ranges if required.


Next to Dual Homing (a mirroring voice resiliency service in case of two sites), Colt offers a few more resiliency options, ensuring your business continuity:

  • Voice resiliency (Inbound call diversion in case of failure)
    • Inbound Call Rerouting Resiliency (preconfigured, automated call diversion in case of network failure, divert to single number)
    • Partial Number Replacement Resiliency (preconfigured, automated call diversion in case of network failure, per ranges)
    • Disaster Recovery Resiliency (preconfigured, invoke/revoke via support desk)
  • Inbound call distribution
    • In case of multiple PRIs, calls can be sent to different PBXs.
    • Colt supports both plain TDM PBXs and hybrid TDM-IP PBX devices
    • A Voice Line (v) service can handle multiple Local Area Codes for a single service country
    • Both sequential and round robin are supported
  • Convergence with Colt IP Access

Number Format

To allow for a good working service and complete CDRs, the Voice Service Details settings should match with the configuration of the customer's PBX.

  • If the wrong (or no) CLI is set up (customer A-number) in the customer's PBX, calls to the emergency services will use the Default CLI, hence the emergency services in the wrong area may be called. To avoid this, the PBX must enable the customers' CLIs to be sent to Colt.
  • If no Default CLI is given by the customer, Colt will set any number of the given ranges as default.
  • The Main Number is set up per trunk (site) and appears in the CDRs (unless a reseller profile is applied).
  • Following Egress Number formats for Called Party Number and Calling Party Number (for CLIs of Incoming calls from customer's perspective), are supported by Colt. Other number formats are supported upon request):
    • DDI (Direct Dial In) – For Incoming Calls (Customer receives a call). The A- and B- number can be received in the following formats:
      • National (NSN, TON=NAT)
      • National (0 NSN, TON=UNK)*
      • International (CCNSN, TON=INT)
      • International (00CCNSN, TON= INK)
      • Extension Digits (TON=UNK)
      • Subscriber (SUB, TON= SUB)
    • If the Egress Number Format Colt configures does not match with the customer’s PBX, incoming calls will fail.
    • If not provided, default settings are NSN (TON=NAT) and 0 NSN (TON=UNK)*.
    • If the number format for customer's outgoing calls is not supported, only the Default CLI will appear in the CDRs, and not the DDIs with the extension of the actual caller.
    • * In countries where national dialling prefix is used
  • For outgoing calls (customer places a call), the A and B number can be sent in any of the following national and international formats to allow Colt to recognize the numbers.
    • DDO (Direct Dial Out) – For Outgoing Calls (Customer places a call), the A and B number can be sent in the following formats:
      • A Number (Originator):
        • NSN (TON=NAT)
        • 0 NSN (TON=UNK)
        • CC NSN (TON=INT)
        • 00 CC NSN (TON=UNK)
      • B Number (Destination)
        • NSN (TON=NAT)
        • 0 NSN (TON=UNK) (In countries where national dialing prefix 0 is used.)
        • CCNSN  (TON=INT)
        • 00CCNSN  (TON=UNK)
        • CCNSN (TON=INT)
        • 00CCNSN (TON=UNK)
  • With matching formats between both the customer’s PBX and the Colt configuration, Colt can show both main and A-number (caller) in the CDRs.


  • NSN – National Significant Number : This is the telephone number without the initial ‘0’ trunk code or the ‘00xx’   country code. Significant digits without leading zero (e.g. For dialled number: 0170996465; the NSN is : 170996465 and 0+NSN is: 0170996465).
  • CC – Country Code : digits defining destination or origin country of the call (e.g. for dialled number 0033170996465; the CC+ NSN is: 33170996465 and 00+CC+NSN= 0033170996465)
  • TON= ISDN Type of Number : National (NAT) / International (INT) / Unknown (UNK)

Valid CLI

It is increasingly important to ensure that you provide a valid CLI (Calling Line Identity) for outbound calls.   Regulators and many carriers are tightening their rules around handling and charging of calls based on the calling number (CLI) for the call. This is due to a changing regulatory and commercial environment where call integrity and charges are based upon the calling number.

As some terminating carriers are now blocking, charging at a higher rate or changing the CLI this may impact calls from Colt customers.

A valid CLI needs to meet the following criteria:

  • Begin with a '00' in global format
  • Contain a valid country code
  • National dialable number
  • Consist of the correct amount of digits (e.g. for UK: no more than 12 digits including the 44 code)
  • You must be entitled to send the CLI (have the right to use the number as an originating identifier)

Any call sent to Colt with a CLI which is not conforming to these criteria may have the CLI changed to the Default CLI (which is part of your number range) or potentially the call may be blocked.


Valid CLI - CLIP No Screening - Presentation Number

Colt recognizes the importance of the phone number you wish to display for outgoing calls. Therefore, Colt supports number presentation functionalities ‘CLIP No Screening’ and ‘Presentation Number’, where regulation allows. However, Colt cannot guarantee the functionality across other carrier’s networks. Furthermore, Colt has the obligation to exchange a valid and compliant presented CLI (Calling Line Identification) between different operators (ie “valid and compliant” means following format and rules of national numbering plans).

Local regulatory bodies and authorities that govern the provision of telecommunications services nationally and at European level are tightening the rules around handling and charging of calls. Customers must comply with the relevant national numbering plans and the ‘CLIP No Screening’ and ‘Presentation Number’ usage conditions which Colt has summarized in the ‘CLI Presentation regulatory requirements per country’ table, available at the following link:

In case of invalid and/or uncompliant presented CLI, Colt has the right to suspend or withdraw a service, including the use of the ‘CLIP No Screening’ & ‘Presentation Number’. Customers must therefore ensure calls are sent with a valid and compliant presented CLI and manage the presented phone number accordingly in the PBX.

To prevent an interruption of your service and to accommodate Colt’s obligation to provide a valid and compliant presented CLI, Colt will replace any invalid and/or uncompliant presented CLI, independent from number presentation option, with the ‘Default CLI’ defined per service, which is always a valid number configured on your service.

To change the ‘Default CLI’, a modification order is required.

Unsupported features

  • National voice protocols: DPNSS & DASSII (UK), …
  • Supplementary Services MoU 2 ( CF (Call Forwarding)*, AoC (Advise Of Charge), MSN (Multiple Subscriber Number), 3PTY (Three Party), CONF (Conference Call Add On), UUS (User-to-user signalling), …
  • * Colt supports CLI handling, thus displaying the original calling number instead of the redirecting number, in case the customer configures the call forward on his PBX.
    • Colt ‘Network based’ call forwarding can no longer be supported on VoIP Services: Unsupported Partial Re-routing facility method (ISDN supplementary services) - Method for call forwarding calls on busy, no answer or all calls (unconditional)
      • Colt recommends to optimise the configuration of your PABX: Calls have to be forwarded from the PABX, forwarding on a DDI number within the PABX.
      • To enable Call Forwarding, a change needs to be implemented in your PABX by your PABX maintainer. Colt has no access to this device and cannot execute this for you.
      • Alternatively, if you require call forwarding for resiliency reasons, we recommend our inbound call resiliency options or the configuration of a separate Colt IN Geo service, or you can order a particular 'Partial Number Replacement' feature on the service that will do call forwarding
  • Videotelephony, Trading, Channel Bonding
  • (BRI (Basic Rate Interface) - upon request only)
  • Point-to-Multipoint BRIs and TP (Terminal Portability)
  • Analogue lines (POTS), Lift (elevator) alarm, Health alert, …
  • Fax: Group 4 (56.6k)
  • Modem: V.90, V.92 and V.23
  • Point of Sale: POS devices with modems operating with an answering tone other than 2100 Hz only (V.25 modem tone)
  • Clock pulse: primary reference clock

Colt cannot offer an alternative for data calls, bonding, alarms, … and recommends to seek specific solutions with enterprises who can offer services based upon mobile 3G, wifi, … technology. In general, other operators including national incumbents (PTTs) tend to stop supporting analogue lines etc.

Migrating to Voice Line (v)

  • A Colt technician will require access to your premises as the migrations require physical installation work prior to the migration. The Colt technician can indicate where the new connection needs to be made by the customer at the time of the migration. Therefore it is advised the customer is on site together with the Colt technician and takes note of the new connection point ready for the cut over.
  • For the actual migration, Colt needs a two hour window in the early morning or during business hours. We require you to be on site to plug in the connection to your PBX (Voice Gateway installation).
  • Scheduling: The Switch engineer informs the customer via mail and phone call about the start and end of the activity.
  • Outage duration: The migration of numbers takes minimum 10-15 minutes for the customer to move the patch to the new connection while Colt makes the changes in the network. In case of many numbers/ranges, more time will be required to synch. Another 45 minutes may be required to allow you to test the service, or to roll back. Colt will perform the migration remotely and may not send a technician on site to support.
  • Incoming call - test call: (Before and) after migration to confirm service status. The customer is not required for Colt test calls but needs to be available to confirm the migration went ok. Colt recommends the customer to place a few test calls.
  • Voice Resiliency services such as Disaster Recovery, Partial Number Replacement and Inbound Call Rerouting can only be tested after the delivery of the base service.
  • Project Management: For standard migrations, Colt foresees no project manager per service, but support can be obtained via your Account Manager.


Colt aims to visit the customer’s premises only once to do the physical installation of the service. In case a 3rd party OLO is involved (for Leased Line and wDSL Access Types), the 3rd party will do its installation prior to Colt’s installation.

Colt ensures the customer is notified about the visiting technicians so that the customer can organise its availability.

Colt’s installation does not include internal cabling at the customer’s premises (or cross connect cabling in a carrier hotel or datacentre), unless this is ordered additionally.

Colt’s demarcation point is the Voice Gateway. Colt delivers interfaces and the customer connects his equipment to Colt’s equipment after this demarcation point. In case a Colt Presentation Panel is installed, Colt’s demarcation point is this Presentation Panel.

Patch cable - Connector

The customer patches the PRIs from his PBX to the Colt Voice Gateway or the presentation panel using ISDN network cables equipped with RJ45 connectors. ‘Straight’ cables are required.  In case of a COAX cable for a PRI, a balun (balanced-unbalanced) convertor would be required. The ports on the voice gateway device are configured for NT mode.

In case the ISDN service is delivered over an internet access provided by a company other than colt, the customers should connect it to the Colt voice gateway with another ‘straight’ ethernet cable.

Customer Premises Equipment (CPE)

Colt will install the required devices at the customer’s premises and will manage and maintain those.

For each Voice Line (v) service, one or more Voice Gateways is needed, depending on the number of lines (One per 2 PRIs in Europe and one for all PRIs in Japan or two in case of 9 or 10 PRIs).

The rest of the equipment is rather specific to the Colt access service bearing the voice service and depends on the Access Type: Colt or the OLO will require to place a modem. On top of that, Colt will also install the right physical router, depending on the bandwidth requirements, for Converged Off-Net deliveries.

Per default, these boxes are not rack mountable and may require a shelf in a rack. The required space depends on the type of service. Each device takes up less than 2 U.

In case the ISDN service is delivered over an internet access provided by a company other than colt, Colt will only deliver a preconfigured voice gateway which the customer should connect to his internet and the PBX devices. To enable a quick test, customer readiness is expected.

Prerequisites Voice Line (v) over public internet
The customer's connectivity should be compliant with following conditions:

  • The customer's broadband router should support QOS, COS and IPSec passthru and have one DHCP enabled LAN port available for Colt s Voice Gateway per max. 2 PRIs.
  • Colt cannot manage the customer’s router. In case of issues with the internet connectivity, the customer handles the communications and resolution with the ISP.
  • In case of convergence, the minimum bandwidth capacity is the double of IP bandwidth required for voice traffic, otherwise the minimum requirement is the IP bandwidth required for Voice.
  • In case a capacity of more than 2 PRIs is required, a bespoke solution with POC is due as not all routers are equipped to support the service.
  • UDP ports 500 & 4500 should be enabled to all the IPSec protocol to build the encrypted tunnel from the customer s router to the Colt target routers.
  • In case of Firewall, to allow the traffic to pass through, the customer will have to whitelist IP Addresses of the Colt target routers &

To allow Colt’s customers to retain a TDM PBX for ISDN telephony, Colt deploys voice gateways per site to allow the TDM signaling to be converted to IP (and vice versa).

Examples of used Voice Gateways

OneAccess ONE700 devices are generally used for installations with 1 or 2 PRIs. Other devices may be installed depending on required interfaces.