A call from a Public Switched Telephone Network (PSTN) through a gateway to a Cisco CallManager IP phone, Cisco IOS gateway, or third party H.323 device, may not get a busy tone when running either an application or two-stage dialing on the originating gateway.
Note: Two-stage dialing refers to the calling party dialing the number to the gateway first, receiving the dial tone, then dialing the called party.
In either case, the call has connected in terms of the PSTN network once it terminates on the originating gateway. If the IP call leg comes back with a release with the user busy cause, the message cannot be indicated back toward the Telephony session that is in a connect state.
This issue has been addressed by having the originating gateway generate a busy tone when the release from the IP call leg is received with a cause code of user busy. The telephony leg will either be released by the calling party or by the gateway after several minutes with the cause code of normal call clearing .
This feature is available in Cisco IOS Software Release 12.2(8)/12.2(8)T and later.