The source phone sends a 180 Ringing signal to the Cisco UCM. which sends a 180 Ringing signal to the destination phone. The same process applies to 11 BYE messages.

The source phone must signal to the destination phone that it is ringing, and the destination phone signals back with a 180 Ringing message. The same process applies to 11 BYE messages.

The calls have an MTP in the call path due to different codec support. The calls are subsequently split into two call legs.

The destination phone signals back to the Cisco UCM that it is ringing, and the Cisco UCM signals back to the source phone.