This is not normal behavior for the HTC Rhyme and we want to make sure to get it addressed. I want to find out if this problem still persists. If so, is anything covering the microphone? Do you have any connected accessories? Is this calls with everyone or just certain people? What happens with the talk to text function? Do you get an error message or does it not open? I look forward to hearing back so we can get this fixed.
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I had that problem the other day. What remedied it was instead of hitting "power off" on the phone, you touch "restart". This shuts down all the apps and restarts the phone. It will then work for you.