I'm having a problem along the same lines. I moved to Tmo a month or so ago when the S5 came out (ported from Sprint). The first couple of days I had great coverage but intermittently when I talked to someone, they'd tell me it sounded like I was in a tin can. At times it was hollow or echo-y. It kind of stopped, but then started up again. To me, everything sounds perfect. Now, randomly when I talk to someone, sometimes they can hear me without any problem, and sometimes they can barely understand me because of so much feedback coming through (even though I can hear them perfectly).
Tmo gave me a new sim card to use and said to wipe my cache to make sure everything was good. I did that and it worked great for about a day. Now, it's random again. I call land line and it sounds like I'm in a tin can and when they talk it's lots of feedback. I call friend on att and they have loads of feedback when they talk but perfectly fine on my end. I called someone on Tmo and had the HD appear on the call and it sounded like he was talking through a pillow.
It's so weird I can't figure out what's going on, but it's getting really annoying trying to talk to someone and end up calling them multiple times just to see if I can get a clear connection.
Bluetooth in my car seems to work perfectly for each end, though.
I'm on stock rom, rooted w/ philz recovery, and have not updated to the new ANES.
I don't use a case.
Wondering if that would help out.
Sprint | HTC Touch > Hero > EVO 4g LTE
T-Mobile USA | Samsung Galaxy S5
Recovery: Phil'z w/ StockRom