Blind transfer unsuccessful - W52P and Cisco Call Manager 9.1.2

[fidotas]

I'm attempting to get a Yealink W52P working with Cisco's Call Manager 9.1.2. Everything is working as expected except for call transfers. The scenario is fairly basic:

Phone A calls the Yealink W52P who transfers to the call to Phone B.

I've also confirmed it occurs when the Yealink W52P initiates a call to Phone A and attempts to transfer the call to Phone B.

A call transfer is initiated either as a "blind transfer" where with an active call in progress on the W52P handset we press the Options key then Transfer, enter the destination number (72038) and press the Transfer key again to complete. Or a transfer is initiated as a "consultative transfer" where with an active call in progress on the W52P handset we press the Options key then Transfer, enter the destination number (72038) and press the Green Speaker button to initiate a second call. The second call connects successfully, we speak with the person on Phone B then press the Transfer key on the W52P handset again to complete the transfer.

"Blind transfer"s fail completely. The W52P indicates the transfer is complete with a "Call transferred" message on the display (I can see in SIP traces the correct exchange of REFER, ACK and NOTIFY) however it plays an audible BEEP BEEP BEEP indicating the call has terminated and the call to User A is terminated. User B never receives any incoming call notification.

"Consultative transfer"s work. The two call legs are successfully bridged and User A can speak with User B. However the W52P handset emits an audible BEEP BEEP BEEP indicating the call has terminated despite displaying a successful message.

Are you able to assist with getting blind transfers working? Removing the incorrect audible alert would also be great as it will confuse handset users making them believe the transfer was unsuccessful.