5.6 Optional Configuration
5.6.5 Called party number answering

Multiple devices can be physically connected at your site. You can ensure that only a single device answers an incoming call by verifying the number or subaddress in the incoming call against the device configured number, subaddress, or both.

ISDN: SAPI and TEI
The SAPI and the TEI taken together create a unique identifier for a specific logical link, which, in turn, represents a specific TE. The TEI is a 7-bit number carried in the address field of the LAPD frame. Although the TEI 7-bit code allows up to 127 devices on an S-bus, there are limits to the number that you can attach: up to eight devices with the 5 Electronic Switching System (5ESS) and NI-1 switches, and up to two devices for the DMS-100. Configuring multiple devices on an ISDN line is referred to as ISDN multipoint.

You can specify that the router verify a called-party number or subaddress number in the incoming setup message for ISDN BRI calls, if the number is delivered by the switch. You can do this by configuring the number that is allowed. The following is the interface command that provides a verification of the called-party phone by using the number supplied in the call setup request:

Router(config-if)#isdn answer1 [called-party-number][:subaddress]

If you wish to allow an additional number for the router, you can configure it with the following command:

Router(config-if)#isdn answer2 [called-party-number][:subaddress]

If you do not specify the isdn answer1 or isdn answer2 command, all calls are processed or accepted. If you specify the isdn answer1 or isdn answer2 command, the router must verify the incoming called-party number and the subaddress before processing or accepting the call. The called-party number should not be mistaken for the number used by the router to initiate the call.

Lab Activity 
  You have other ISDN devices connected to the same ISDN line that your central router is connected to (this is possible with an S/T Interface). To ensure that they don't step on each other, we want to configure the central router to only answer calls that are placed to the 5554000 number.