AT&T not invest in infrastructure totally untrue. I'm in NYC the company I work for builds Data centers for verzion and AT&T. AT&T spares no expense when it come to infrastructure. AT&T is the only carrier that builds iPhone specific data centers. I have seen 2 year old data centers get ripped out and replaced. Verzion rents a couple of floors in the AT&T building I work in. Ima tmobile user.I don't mean to sound cynical, but calls on AT&T already sounded amazing on my old iPhone four years ago, but have steadily degraded over the years.
It's probably network congestion, but I can't help but feel I had great voice quality before, lost it because AT&T didn't invest in their infrastructure and now I'm being sold it again (no doubt at an additional cost).