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Cisco CallManager 5.x does not receive a ring back for PSTN calls with SIP
The annunciator is used for Session Initiation Protocol (SIP) ring back. So, the SIP Trunk needs to have a media resource group list (MRGL) that contains the annunciator or the annunciator needs to be in the default list. Otherwise, no ring back is received.
In order to fix this issue, add the existing annunciator media resource group (MRG) to all the remote MRGLs.
Cisco CallManager uses Skinny Call Control Protocol (SCCP) messages in order to establish a Real-Time Protocol (RTP) stream connection between the annunciator and the device. The annunciator plays the announcement or tone in order to support these conditions:
Announcement Devices configured for Cisco Multilevel Precedence Preemption
Barge tone Before a participant joins an ad hoc conference
Ring back tone When a call is transferred over the Public Switched Telephony Network (PSTN) through a Cisco IOS gateway
The annunciator plays the tone because the gateway cannot play the tone when the call is active.
Ring back tone When calls are transferred over an H.323 intercluster trunk
Ring back tone When calls are transferred to the SIP client from an SCCP phone
So the annunciator can play a ring back tone, a call is transferred over the PSTN through a Cisco IOS gateway.