T46G not registering with AVM Fritzbox 6360

[t-bbc]

Hi,

I also currently bought a T46G (firmware 28.72.026) in order to use it with a AVM Fritzbox 6360

In difference to frame_michael I am not able to connect directy to the fritzbox but via two airports from apple (one serving as the DNS router, one working as a bridge).

I am able to connect soft phones but not the T46G.

I used the following settings

Register Name 623 (same on FB)
User Name 623
Password as registered on the FB
Outbound Proxy disabled
Transport UDP
NAT disabled

SIP Server 1
IP-address of fritzbox (which works for soft phone)
Port 5060

In addition, forward the UDP Port in the airport serving as DNS Router
to the IP of the T46G

But the registration does not work.

Unfortunately, I don't get any error messages either on the T46G nor on the FB.

Please, can you give me a hint what went wrong?

Best Regards, Ralf