t26p don't transfer

[ESSEF]

I have a similar problem since about a week: When someone tries to transfer a call to my phone (and also to one other phone of a colleague) they get the message 'Transfer Failed'.
We use 3CX for about two years without problems and the upgrade to version 12 was a few months ago. Since then nothing was changed.

I was suggested to upgrade the firmware of the phones to 6.71.1.1 but the strange thing is that the firmware upgrade runs without any errors but afterwards the firmware version remains 6.71.0.168. Any ideas?

Thanks in advance for your reply.

P.S.: All the phones (10) are Yealink T26P.