If you manually forward phone A to Phone B and then manually forward - no answer, Phone B to voice mail, the system will always send the call back to phone a's mailbox. It has always worked that way.
The only way you can provide a solution for the customer is to create a CRA for each phone, Make the CRA time out to that extensions mailbox. Then Phone B can forward- no answer to their CRA.
Example = ext 110 has CRA 2710, ext 111 has 2711.....