Arriving calls mutely with VOIP over UMTS

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Hello, I has 7240 with the firmware with the L.G. version 73.04.97-14138.

My Sipgate Voip number is registered in the Fritz box.
Arriving calls from the fixed net/mobile phone ring at the MT-D and also in the call list are indicated. If I take off, the other party can hear me, I hears however absolutely nothing. Exactly the same if I call someone, I do not hear a bell signal although it with the other one ring. I do not hear thereby the interlocutor, he me however.

This problem does not have I if I a VoiP number select. For the Internet entrance use I mean UMTS stick of Huawei.

For the definition of Sipgate in the Fritz box I cannot indicate a STUN server. Evtl is the problem? I can amusing-prove also no haven releases to make, since with the IP this menu option is missing to attitudes.

Can the problem be solved somehow?
Thanks and greeting