A Basic Rate Interface (BRI) line may have as many as eight devices attached.

All these devices may use the common D channel at the same time. For example, one device may be trying to make a call at the same time as a call is being received and answered by another device.

The ISDN network therefore needs to be able to conduct 'conversations' with each device independently.

To do this successfully, each device requires its own address, called a Terminal Endpoint Identifier or TEI. This address is not the telephone number. Telephone numbers belong to the line, not to any device attached to the line.