Registering with FritzBox 6490

[Boas]

Hi,

I'm trying to register the W52P to my Fritzbox 6490.
Unfortunately, I does not work

Setting from Fritzbox
Registrar: fritz.box
User: 620

On the webinterface of the W52P I tried it with
Registrar: fritz.box
User: 620
Using 620 also for Display name, account name.
As SIP Server 1 I entered: fritz.box

Setting I also tried, are:
- using IP instead of DNS name for SIP server
- using 620 also as registrar name (as mentioned in another thread here dealing with another fritzbox version)
- using 621 instead of 620 (also as mentioned in other thread)
- creating and using 621 user on fritz box
- and various combinations of it...

I also updated the firmware of the W52P and did all tests again, but still it didn't work!

Has anybody expirenced the same problem? Or got it to work on this fritz box version? Or any other advices?

Greetings