While on a call keeps dropping out or the sound is garbled.

While on a call keeps dropping out or the sound is garbled.

I have T-Mobile minutes that I buy in $100 increments. I just bought a block and this drop out problem happened. I tried to put the sim card in an identical model, but got it to ring with my number, but could not change who it was given to. Can I go in and change all of the information?