No voice on a regular basis (Caller/Callee can't hear my voice)

[Stati]

Hello,

slowly I'm dispairing of my T46G. Every few calls my opponent doesn't hear me. I hear them always loud and clear.

Regardless whether I'm calling/answering with handset, headset or hands free, my voice can not be heard. After rebooting the phone, everything works fine for a few calls, but then the problem starts again.

I received the phone with firmware 28.73.0.40 and I operated it for about one week without problems. When my headset arrived (Yealink YHS 32) I updated the phone to the newest firmware 28.73.0.50. I don't know the exactly when the problems started, but it was about that time.

I reset the phone to factory settings, I downgraded to firmware 28.73.0.45, operated with headset plugged in and without, but with no improvement.

I am operating the phone at Fritz!Box 6360 router, provided by my cable provider Unitymedia. I am using the Fritz!Box as SIP-Server and all expected features work as expected.

Would be great if somebody could help me to solve the problem.

Thanks in advance

Stati