Unable to Transfer Calls on 7900 Series Phones

Recently encountered an issue in 7900 series phones where transferring a call when using a headset doesn't work, this is a confirmed bug in firmwares 9.3(1) SR4 & 9.3(1) SR4.1, full details here. The symptoms are:

• When a user attempts to transfer a call, the original call is placed on hold, but the user is not able to dial the transfer destination.
• When a user already has an active call on their phone, they are unable to answer a new incoming call. The original call is placed on hold, and the new call is simply disconnected.

Unfortunately every device pack for CUCM since around December 2013 includes affected firmware, the solution is to roll back to an older firmware or request the engineering special firmware from TAC.