For reference, these both are new, unlocked AT&T-branded phones running the same T-Mobile firmware. I have verified with the "Field Test" that both are receiving Band 12 well.
You flashed a T-Mobile firmware on an AT&T phone?
I don't know if you'll be able to get much support for that. There's a chance some hardware piece is different between the two that's causing it to not fully work?
Unfortunately, we wouldn't really have a good guess as to why it is is happening on one, and not the other. It may just be something going on with the hardware or software, but realistically that doesn't help us narrow down much for a solution. You could try a master reset on the device, but I am not sure that would actually be any sort of a fix. There isn't any setting that would change the quality of the voice or anything else that needs to be enabled.
Thanks for the replies. I can't imagine what hardware issue could cause this as it seems to be a transmission problem with just one of the phones since it affects both ends of the call. I was hoping that it was just a "setup delay" issue for the new line like the problem I had with visual voicemail that resolved itself a few days after activating the new line. But that doesn't seem to be the case. I hope this will magically be fixed when we (hopefully) are allowed to upgrade to Windows 10 Mobile.