Yeah so far (Only a week) with T-Mobile I've found the call clarity to be pretty darn good.. I can't tell any difference from when I was on Sprint before....
I also can't tell any difference in voice quality between Sprint and T-Mobile.
When on Sprint, I never had any issues with call drops, but constant data issues and text messaging issues were normal.
Haven't had any dropped calls on T-Mobile either, but I don't make but about one call a week, anyway

but so far I'm very impressed with T-Mobile....
I have been extremely happy with everything about T-Mobile so far, EXCEPT being text spammed by them, which is infuriating and I wrote multiple letters to their support and corporate offices about it. (Sprint never did such crap). They say they "can't" turn it off, which is utter bull-crap. They "can" and choose not to.