T48G not registering on Fritzbox 7360

[Lion237]

Hi,
I have problems registering the T48G on a Fritzbox 7360. I already updated on firmwares on both sides on newest versions.
I also tested a softphone on the Fritzbox, it works without any problems.

register name: 620
user name: 620
Password: checked
Outbound Proxy server acitvated: 192.168.178.1 (=fritz.box)
Transport: UDP
Server Host: 192.168.178.1, Port 5060

Telecom provider: Deutsche Telekom, IP connection
I have also registered some DECT phones to the Fritzbox, which work perfect.

Anything wrong with this configuration? Please give me advice, how I can register this phone.
Best
Franz