Since getting it away from my fat cheeks
I haven't had any complaints.
Be sure to try the other idea posted above. Something about holding it near the bottom may conflict with the antenna.
If possible, try a BlueTooth headset as that should eliminate both of these potential problems.
The cause might be that you are in an area with poor antennas on T-Mobile's part. Obviously, it this is the cause and you plan on being there quite a bit, you may want to change to a different provider. Hopefully this isn't the reason.
You could also try the coverage map on T-Mobile's site to see it you supposedly have good coverage at your location(s).
Lastly, you could try T-Mobile support for additional ideas.