T20 can not dial out

[dede]

Hi James,
as a test I unplugged the Netphone KE1020A and used its account for the YeaLink T20P.
No change: outgoing calls are not working.

My "server" (the FritzBox) is a router-box and there is no real log for telephone functions.
But there is a callmonitor.
For the KE1020A it shows:
25.03.15 15:04:41;CALL;3;20;04xxxxxxx06;01xxxxxxx49#;SIP0;
25.03.15 15:04:50;DISCONNECT;3;0;
...and my cellphone was ringing.

Doing the same with the T20P it shows:
25.03.15 15:07:22;CALL;1;21;04xxxxxxx06;01xxxxxxx49#;SIP0;
25.03.15 15:07:34;DISCONNECT;1;0;
...and my cellphone did not ring.
If my cellphone is near to another telephone, I can hear, that my cellphone is being contacted.
Normally, it rings a second after I hear this sound.
If I call my cellphone (laying next to the other phone) with the T20P, I hear the sound, then
the sound stops and the T20P brings the "line busy"-sound.

So...because of this behavior, I think its a protocol related problem.
The call goes out, the called device is being contacted but the session could not be established.

Detlev