If it were my system, I would first turn on Protocol Capture under the Diagnostic settings for the Phone Trunks involved, place the call, then download the logs from the Edge to see what the SIP messaging says.
It's possible there is a protocol or codec mismatch between the phones. Are they different types of phones? Are the phones registered on different edges?
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George Ganahl CCXP, GCA
Principal Program Manager
Genesys
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