I think we can send some commands to Cisco IP phone in form of XML to stop call back ring tone.
I did it 7-8 years ago.
I am not sure whether that is available now also.
Have you found a solution for this?
I think I have the same problem as you except for the part the the phone being called should also get it's ringer disabled.
U can disable called phone ring using Axl. When there is call alerting event send XML for volume 0 to called phone using send data.
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