The ISDN telephone network
Suppose that we want to realize a telephone communication network between two ISDN telephones that are connected to two different "S" buses of the same telephone exchange.

The two different possible solutions refer to the way the telephones are used:
  Telephone in single number mode
 :   in this mode the telephone answers to all the incoming calls on the S-bus to which it is connected.
Telephone in multi number mode
 : in this mode the telephone answers only to the incoming calls relevant to its own number.

The working situation is shown in the following figure:
  In the case of a T1 telephone that is programmed in Single number mode, it will answer to all the calls relevant to the following numbers: 19, 20 and 21.
In the case of a T2 telephone that is programmed in Single number mode, it will answer to all the calls relevant to the following numbers: 22, 23 and 24.
If the two telephones are programmed in multi number mode (MSN = Multiple Subscriber Number), they will answer only to the programmed number.
For programming the telephones in single number/multi number mode, see the documentation relevant to the devices DL TC73-DT.