When an off-net call is attempted through the Session Initiation Protocol (SIP) trunk, the caller gets a fast busy tone. All the incoming calls to the IP phones are fine.
In order to resolve this issue, first make sure that these settings are in place:
There are enough Media Resources available.
The Cisco IP Voice Media Streaming Service is started. If possible, restart this service.
Verify that the proper codecs are being used.
Then, reset the Software Media Termination Point (MTP) in the Cisco CallManager.
Check the protocol configured under System > Security Profile > SIP Trunk Security Profile for outgoing and incoming calls. By default, Cisco CallManager tries Transport Control Protocol (TCP) on outgoing calls. This might time out as it is not able to establish a TCP connection with the SIP gateway. If the protocol is set to TCP, then change it to User Datagram Protocol (UDP).