I recently started using a Yealink T42G - An older (but still very functional) SIP Desk Phone. Weeks passed without any issues, until I attempted an attended transfer. I'd receive a call, chat to them, hit transfer, dial the next number, hit send, and my desk phone would start ringing again displaying "ReCall" on the screen. If you answer the call, the leg to the phone you wish to transfer to will drop.
Fortunately I was eventually able to find a fix. I presume the same fix will work on the other T4X series phones, including the T40, T41 T46 and T48