The object specifies the CAS group number of a CAS
capable T1/E1 that is in the dialCtlPeerCfgLowerIf object
of RFC2128.
This object can be set to a valid CAS group number only if
the dialCtlPeerCfgLowerIf contains a valid ifIndex for a
CAS capable T1/E1. The object must set to -1 before the
value of the dialCtlPeerCfgLowerIf object of RFC2128 can
be changed.