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Using ISDN2 and ISDN2e | BT Business

Using ISDN2 and ISDN2e

A single, high-quality, ISDN2e line has two 64k channels. Usually you’d use these separately, so you can make two calls at once. These could be two simultaneous voice calls, two data calls or one voice call and one data call.

You can also add more ISDN2e circuits to your installation, increasing the number of simultaneous connections and/or available data bandwidth.

Charges for voice calls are the same as for analogue lines, as are data calls within the UK. There are different rates for international 64k data calls.

For more information on how ISDN2/2e works and getting started, go to our Getting started with ISDN2 and ISDN2e guide.

ISDN User Guides

For more information on ISDN2e:

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Differences between point-to-point and point-to-multipoint configuration

The main differences between point-to-multipoint (Standard Access or S/T reference point) and point-to-point (System Access or T reference point) configurations are:

1. The number of devices that you can plug into the box on the wall:

  • Point-to-multipoint allows you to connect up to eight separate devices to the box on the wall.
  • Point-to-point allows you to connect just one device. This is usually a PBX.

2. The type of telephone number range that you can use:

  • Point-to-multipoint allows up to eight MSN telephone numbers.
  • Point-to-point uses DDI numbers, which come in blocks of 10. There is no maximum to the number of DDIs you can have.

Features and benefits of ISDN2e explained

Help using features of ISDN2e.

Diverting an individual DDI number on your ISDN2e line

You can use Customer Controlled Call Deflection (CCCD) to divert calls to individual DDI numbers. This is mostly equipment-based but must first be enabled on the line. Each call is offered to your equipment individually. You then choose to answer the call or deflect it to a specific number. You will be charged for the diverted leg of the call. In order to control the call deflection service, you need a suitable PBX or switch. To order CCCD, call Business Sales on 0800 800 152.

Presenting a DDI number in place of your outgoing CLI

There’s not an exchange-provided service. You need a PBX or switch that enables you to allocate each DDI number to a phone extension, so that incoming calls to DDIs are routed appropriately and have a corresponding number presentation.

Options for forwarding calls made to your number

There are two ways of implementing call forwarding on ISDN2e:

  1. Admin Controlled Call Forwarding
    Admin Controlled Call Forwarding (ACCF) is an exchange-controlled call forwarding option. This means that it doesn’t require special equipment. To set up this service call Business Sales on 0800 800 152. The service is active within 24 hours.
  2. Customer Controlled Call Forwarding
    Customer Controlled Call Forwarding (CCCF) diverts calls to all numbers to a single number of your choosing. This call-forwarding option is applied by you by programming your equipment to divert the calls. This means it’s exchange-independent. Contact your equipment manufacturers if you are unsure whether your equipment supports the service. To enable call forwarding on the line, ring Business Sales on 0800 800 152.

    The forwarded leg of the call is chargeable. The charges appear on your bill along with the subscription for the forwarding service.

Options available for both Admin- and Customer-Controlled forwarding:

  • Call Forwarding Unconditional (CFU)
    All incoming calls are immediately forwarded to the nominated destination number.
  • Call Forwarding on No Reply (CFNR)
    All calls are forwarded to the nominated destination number if the call is not answered within approximately 20 seconds.
  • Call Forwarding on Busy (CFB)
    All calls are forwarded to the nominated destination number if the line is engaged.

If you have an ISDN2e line with MSNs present, the call forwarding is configurable on a per MSN basis. This isn’t possible with admin-provided call forwarding, and also not possible if you’re using DDI numbering.

The difference between Customer Controlled Call Forwarding (CCCF) and Admin Controlled Call Forwarding (ACCF)

The two services do the same thing, by diverting all your numbers to a single number of your choosing. But the way in which the divert is applied differs greatly:

  • ACCF requires a call to Business Sales on 0800 800 152 to specify the number to which you want all your calls diverted. Calls are diverted from the exchange and do not require you to have compatible equipment installed at your premises.
  • CCCF diverts all calls to a single number of your choosing. This again diverts calls from the exchange, but rather than having BT turn it on and off, you can control it from your equipment. This service has to be enabled on the line in order for your equipment to communicate with the exchange and divert the call. Prior to subscription to the service, check with your equipment manufacturers that the service is supported. To order CCCF, call Business Sales on 0800 800 152.

Available call-barring options

There are several types of call barring available:

  • Admin provided Permanent Incoming Call Barring (ICB)
    This lets customers have outgoing-only lines. All incoming calls are barred. Permanent ICB applies to both voice and data calls on all bearer services.
  • Admin provided Permanent Outgoing Call Barring (OCB)
    This lets customers have incoming-only lines. No calls can be made from lines with Permanent OCB.
  • Admin provided Selective OCB
    This lets customers bar certain categories of outgoing calls. These categories are:
  1. all calls (except 999 and 150, 151, 152 154 and 0800)
  2. international and premium rate services
  3. all operator calls (except 999 and 150, 151, 152 154 and 0800)
  4. national, international and premium rate calls (except 999 and 144, 150, 151, 152 154 and 0800)
  5. international, operator and premium rate services.

*Indirect access codes aren’t accepted from the calls barred in this option. Calls prefixed with an indirect access code won’t connect.

Viewing the phone number of the person you’re talking to

You’ll need Connected Line Identity Presentation (COLP) to view the number your outbound call has connected to. This might not necessarily be the number you first dialled.

The service isn’t available when calling PSTN, ISDN2 (DASS), ISDN30 (DASS) or ISDN30e (I421) lines or when the called party has restricted access to their line identity using the COLR service. You’ll also need to have terminal equipment that supports it. You’ll only see the number of your outbound call when the call is connected.

Preventing a caller from finding out the number they’ve connected to

Connected Line Identity Restriction (COLR) stops callers from seeing what number they’ve connected to. It’s available free from Business Sales on 0800 800 152. When you’ve got this, the line identity is restricted to all incoming calls and you can’t active or deactivate this feature on an incoming call by call basis. This service prevents the CLI being viewed by customers that use the Connected Line Presentation service or the Terminating Line Identity service.

Malicious Call Indication (MCID)

You can get Malicious Call Indication (MCID) in certain circumstances, at customer request. This is most likely when there’s been instances of malicious calls and the police are involved.

Sending alphanumeric characters down the ISDN line, along with the number you’re dialling

This is known as sub-addressing and it can be used for calls between ISDN circuits. Sub-addressing lets you send up to 20 alphanumeric characters (apart from the # symbol) when making a call. ISDN users can allocate different combinations of characters to each device connected to their line, in a similar way to Multiple Subscriber Numbering (MSN). Incoming calls that include the sub-address then go to a precise destination which might be a device connected to a LAN, individual devices connected to an ISDN2e line or an application on a host computer.

Before using this service you should check that your terminal equipment can send and/or receive sub addresses. Sub-addressing can’t be used on calls to the PSTN or on international speech calls. The type and location of the call parties determine the maximum number of sub-address characters that can be used.


ISDN2e lines can carry data calls including digital video-conferencing. You can make data calls using a single channel or both channels. If you have a multiple ISDN2e installation, you can use multiple channels (2, 4, 6, 8, and so on).

Upgrading from ISDN2 to ISDN2e

It’s important to check with your switch maintainer before changing from ISDN2 to ISDN2e, especially if you use DDI. ISDN2 supports point-to-multi-point working only. If you upgrade to ISDN2e with DDI, your equipment will need to be reconfigured from point-to-multi-point to point-to-point.

To arrange an upgrade or to find out more, go to our sales pages where you'll find our contact information.

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