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Getting started with ISDN2 and ISDN2e | BT Business

Getting started with ISDN2/2e

ISDN is a digital connection used for voice calls and high-quality data links with other ISDN subscribers. ISDN can be used for transferring large files securely, video-conferencing and linking Local Area Networks at different sites. You can also make data calls to other countries that have ISDN services.

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 help using IDSN2/2e, go to our Using ISDN2 and ISDN2e guide.

Use ISDN2e straight away

You can connect eight digital devices to a single ISDN2e circuit by plugging group four into either socket in your ISDN2e box.

Examples of digital devises you can use:

  • digital telephones
  • digital fax machines or
  • computers equipped with an ISDN card or a Terminal Adapter
  • analogue equipment, which you connect to the ISDN circuit through an interface called a Terminal Adapter.

Terminal Adapters are available from the BT Business Store. Make sure to check that your Internet Service Provider (ISP) supports digital access.

With a different line configuration, instead of connecting 8 digital devices you can connect a digital switch/PBX.

Before connecting a digital telephone system to ISDN2e, please check its documentation or consult the supplier.

Phone numbers

You can have separate phone numbers for telephones or other devices connected to your circuit. How you do this depends on your line configuration:

  • If you connect devices directly to the circuit, you can have up to nine additional MSN phone numbers which you allocate to your equipment.
  • If you use a switch/PBX, you can have an unlimited quantity of DDI numbers.

Exchange services

BT offers a range of digital exchange services for ISDN2e which provide advanced call-handling facilities.

Configuring your service in different ways

An ISDN2e line can be configured in two ways, depending on how you intend to use it:

Standard Access

Also known as S/T reference or Point to Multi-point.

The ISDN2e Standard Access configuration enables signals from the BT network to communicate directly with equipment you connect to your circuit like your telephone or computer. Standard Access is the normal configuration for a single ISDN2e line. You can assign additional telephone numbers to individual devices using Multiple Subscriber Numbering (MSN).

System Access

Also known as T reference or Point to Point.

System Access is required when you connect one or more ISDN2e lines to a switch/PBX. The signal from the network travels as far as the switch/PBX, which then determines where the calls are sent. If you have a switch/PBX, you can use Direct Dialling In (DDI) to associate telephone numbers with particular extensions.


Terminal Adapters

When installing a new ISDN Terminal Adapter, you’ll be asked to select a switch protocol and to enter a SPID number.

Switch protocol

For an ISDN2e line, you can select Euro-ISDN, DSS1 or NET3.

SPID number

SPID numbers apply only to non-European (normally American) ISDN equipment. A card developed for the American market may ask you to enter a SPID during installation.

Equipment asking for a SPID number is not necessarily designed for use with ISDN in Europe and may be incompatible with ISDN2e.

First, make sure that the equipment has the ISDN protocol set to Euro-ISDN, DSS1 or NET3. If it does, it should not request a SPID. If it still requests a SPID, try leaving the key-in box empty or entering your phone number. If this hasn’t worked contact the equipment supplier.


More help

More information on common queries:

Can I receive incoming faxes?

Yes as long as your equipment and/or software supports ISDN connection and is set up right. Contact your supplier for advice.

How can I tell if I have been disconnected from a call?

Most phone systems and Group 4 fax machines clearly indicate call status. Make sure you understand the call-status display to avoid paying for unintentionally extended calls. Also find out how your ISDN card or Terminal Adapter indicates call status.

What is the line loss limit on ISDN2e?

The line loss for ISDN2e is 47 dB. The exact distance this represents depends on the type of cable, number of joints and so on, between the exchange and your premises. For this reason, ISDN installation always needs a preliminary survey.

What signalling type does ISDN2e use?

ISDN2e uses the signalling protocol Q.931 or Euro-ISDN.

Can my alarm system run over my ISDN2e line?

If your alarm system currently relies on a conventional telephone line, you might be able to connect it using an ISDN Terminal Adapter. You’ll need to check with your alarm company that your system is supported on ISDN connections.

How can I present a DDI number in place of my outgoing CLI?

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

I have a new ISDN line and my PBX maintainer needs only 4 digits sent. How can I get this changed?

By default, the exchange sends 6 digits down the ISDN line to your equipment. Just contact us by chatting online or call and we’ll do it for you.




Help getting started with the ISDN box and connected equipment.

Understanding two switches on the underside of the ISDN box

The settings for these switches depend on whether you have any digital extension wiring. One switch has two positions: 'IN' and 'OUT'. The other has two positions: 'S' and 'L'.

‘IN’ and ‘OUT’ switch:

The setting for the 'IN' and 'OUT' switch depends on whether the electrical resistance at the furthest point of your ISDN wiring is inside or outside the ISDN2e box. If you connect your ISDN equipment directly to the sockets in the ISDN2e box without any extension wiring, this switch should be set to 'IN'. If you have digital extension wiring, the termination resistance will be at the final extension socket. The setting should then be 'OUT'.

‘S' and ‘L’ switch:

The 'S' and 'L' switch setting depends on the length of any digital extension installed. It should be set to 'S' for extension wiring up to 150 metres, and 'L' for extension wiring between 150 and 800 metres in length.

In an ISDN2e installation with no digital extension wiring, the switches are set to 'IN' and 'S'.

Compatible PCs

ISDN2e will work with most desktop PCs and Apple Macintosh computers as long as they’re either fitted with an ISDN PCI card or connected to a suitable Terminal Adapter. In either case, the connection must be correctly configured.

To use an Apple Mac with ISDN2e you’ll need a Mac-compatible ISDN PCI card or Terminal Adapter. You might also need additional software for dial-up applications such as internet access or remote access. Please consult your Apple Mac supplier for further details for your particular setup.

Connecting LANs using ISDN2e

If you’re connecting LANs (groups of PCs linked together by a hub device) together using ISDN2e you’ll need to connect the LANs to an ISDN device such as a router at each end of the ISDN2e line. If you’re using a router, the WAN port should connect to the ISDN2e line and the U interface should connect to the hub device.

Using a digital switch/PBX

If you want to distribute calls around the office and use facilities such as call transfer and call pick-up, you’ll need a digital PBX. These systems are also called ISPBXs, switches, switchboards or digital phone systems. You can buy them from the BT Business Store.

Extension wiring

BT can install digital ('S' Bus) extension wiring for an additional charge. You can also buy DIY extension wiring kits from the BT Business Store. Cabling must be Category 5 compliant, conform to ISO/IEC 11801 and be fitted with RJ45 plugs.

ISDN2e wiring must terminate with an RJ45 connector, it won’t terminate with a BNC connector.

With only one piece of equipment, the extension can be up to 800 metres. With more than one, the maximum extension is 150 metres.


Features and benefits of ISDN2e explained

Find out what features and benefits are available with ISDN2e.

BT Call Features available with ISDN2e

There are a range of BT Digital Calling Features (formerly known as Digital Select Services). These include:

  • Call Forwarding
  • Call Barring
  • Calling Line Identity (CLI)
  • Customer Controlled Voice and Data Call Forwarding
  • Call Waiting
  • Call Hold
  • Call Deflection.

Most BT Digital Calling Features are controlled by your equipment. They don’t use pre-set codes such as 1471. You should check with your supplier that your equipment is compatible. To order BT Call Features go to our sales pages, where you'll find our contact information.

The BT Calling Features available with analogue lines, such as Call Sign, Call Minder and Call Diversion, can’t be used with ISDN2e - neither can the BT Answer Service.

Keeping your outgoing number private

You can request that your CLI is revealed to no-one. This service is available free of charge by adding Calling Line Identification Restriction (CLIR) to your ISDN service via Business Sales on 0800 800 152.

If you do have CLIR added to your line you can release your CLI information on a call-by-call basis by dialling 1470 before the outgoing number.

Or if you don’t want to have CLIR permanently added to your line, you can prevent your CLI from being released on a call-by-call basis by dialling 141 before the outgoing number.

Caller ID on ISDN2e

The ISDN equivalent to Caller ID is Calling Line Identity Presentation (CLIP). To order, go to our sales pages where you'll find our contact information. Even though this is an exchange-provided service you need to make sure that your PBX or switch supports it.

Ceased Number Intercept (CNI) on ISDN2e

Ceased Number Intercept can be applied to ceased circuits only. It can be applied to:

ISDN2e on System Access, Point to Point:

  • The main directory number of the circuit.
  • A DDI range.
  • Part of a DDI range. This can be any part as long as it’s in a block of 10 DDIs. As this is expensive, you would normally have just the CNI on the whole DDI range.

You can have a different CNI on each of the DDI ranges and on a different one for the main number. You can decide to have a different CNI on different blocks of 10 DDIs from a range, even if the blocks are from within the same DDI range. However, both options are expensive.

ISDN2e on Standard Access, Point to Multi Point:

  • The main directory number of the circuit.
  • Each MSN - each one is treated as a separate number, so you can have a different CNI for each MSN number.

Remote Call Forward (RCF) on a ceased ISDN2e circuit

Remote Call Forward can be applied to a ceased ISDN2e circuit on the main number, and on any single number DDIs. It can’t, however, be applied to any DDI range or part of a range, even if the main number is part of that range. One option is to have CNI on a DDI range. If the ISDN2e is on P-MP, then you can have RCF on each of the MSNs as well.

For more information on ISDN2/2e including how to use calling features, go to our Using ISDN and ISDN2e guide.


Our Standard Care package is provided as part of the ISDN2e service. You can opt for the Total Care package at an extra cost, through Business Sales on 0800 800 152.

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