Calls connecting but no voice?

I had a period yesterday where my calls were connecting, but there was no audio connection for either party.  I would hear ringing, the destination would ring but neither party could hear anything once the call was answered.   The call timer starts.   This happened for 4 consecutive attempts.  I also disconnected my bluetooth headset and turned bluetooth off.  Wireless was not on. 

This happened for the first time yesterday - 1/7/15.   I rebooted my phone (an HTC One M7) while I attended a conference call on my land line.   By the time I was done, the cell phone was working again.

Today it happened again, however the second call attempt worked fine without a reboot.

Is this a network problem?  Any ideas?