IVR----H323 (call leg 2)-----CCM----MGCP (call leg 1)----PSTN
|------------H323 (call leg 3)---------| Agent IP phone
Call leg 1 and call leg 2 are the incoming call to the IVR, and call leg 3 is a new h323 session for IVR to transfer the call to Agent's IP phone.
In Cisco CallManager 4.x, when an inbound call from a Public Switched Telephone Number (PSTN) hits the Interactive Voice Response (IVR), Cisco CallManager uses one h323 session in order to setup the call with IVR, and when IVR transfers the call to an agent, it sets up a second h323 session to Cisco CallManager. When the IP phone rings, Cisco CallManager sends h225 information to IVR in the second h323 leg in order to indicate ringback on, and IVR passes this h225 ringback information to the first h323 leg. But Cisco CallManager does not convert this information back to the Media Gateway Control Protocol (MGCP) gateway in order to request that the MGCP Gateway play the ringback during the transfer.
The issue is caused because Callmanager is not be able to relay the h225 info ringback on message to mgcp message to request the gateway to play ringback tone while the call of first leg is active.
In order to resolve this issue, change the Cisco CallManager cluster wide service parameter H225 Info for progress tone to Use ANN for Ring Back.
This enables Cisco CallManager to insert the Annunciator (Cisco CallManager Media Resource) to generate the in-band ringback to the caller. As a result, when the IP phone rings during the transfer, the Annunciator in Cisco CallManager generates the ringback tone and the caller hears the ringback.