ISDN CAS ( Channel associated Signalling )

CAS ( Channel associated sgnalling) is a kind of PSTN signalling for E1/T1 Link.

In case of CAS the signalling information is sent over the B Channel and hence the signalling is In Band.
Hence CAS is alos called a "Robbed bit signalling".

Below are the list of signalling used by CAS to detect phone and network events.
1. Loopstart
2. groundstart
3. E&M

The disadvantage of loopstart is that it is prone to glare (A situation where incoming and outgoing call both try to sieze a channel )

The best to use is E&M as it  provide both disconnect and answer supervision and is not prone to glare problem

Typical configuration of CAS include below command:

controller T1 0

clock source line primary

ds0-group 1 timeslots 1-24 type e&m-fgd dtmf dnis
In this case the below type command mean that it card will collect dual tone multifrequecy for DNIS for E&M
"type e&m-fgd dtmf dnis " .

You can find many such frequency or type you can select one as per your requirement....

Good Link from Cisco for T1 CAS troubleshoot..

http://www.cisco.com/en/US/tech/tk652/tk653/technologies_configuration_example09186a00801040bc.shtml