T46G not registering with AVM Fritzbox 6360

[frame_michael]

Hi,
we just bought a T46G (latest firmaware 28.72.0.1) in order to use it with a AVM Fritzbox 6360.
But no matter how I try - I can't get the phone to register with the Fritzbox.

I checked all the FAQs and Forums from Yealink and AVM but without any solution to this problem. I even called support from AVM but they told me it might be a problem with the IP phone.

The phone is connected directly (Lan) to the Fritzbox.
I tried it with DHCP and with a fixed IP adress in the same range as the
Fritbox of course.
I can see the phone in the network of the Fritzbox.
I enter all the necessary information for acount on the phone - but no matter what I try - it always says "register failed".

Does anyone have an idea?
thanks
Michael