Telephony function suddenly failed

When I call another phone(land line or cell) from this HTC, it rings and connects but there is no sound on both phones.
If I call from another phone I immediately get the voice mail message bank telling me to leave a message. My phone never rings or vibrates.

This just suddenly happened. The phone was not dropped.
The speaker works fine, but the speaker phone symbol is greyed out when I am connected.
I have verified the correct function of my SIM in another cell phone and I have tried resets and battery removal. The HTC works well with all other functions, GPS, Wifi, games, etc.
I have not installed any new software in the past 6 months.

I would also appreciate hints where to look if this is a hardware issue.