I have a T-Mobile Simm in my TC75.   I'm not sure if it is a T-Mobile issue or TC75 issue.  On numerous occasions I have had to hang up my call on my TC75 and call from my Samsung Galaxy simply because the call quality was terrible. The other person said I sounded like I was far away.  Sometimes the call would be such that I could hear every other word.  I have gotten to the point, where I have stopped forwarding my calls to the TC75 because of the number of times I have had to hang up and use my Galaxy S4.

Not all calls have been bad, but there have been more bad calls than good calls.    When we don't have call quality issues, the sound and volume are great.

I have noticed that I have more bars on my Verizon Galaxy S4 than my TC75.