Apparently with no fanfare, AT&T has launched VoLTE (Voice-over-LTE) in Dallas, along with what they're calling HD voice. This service allows for fast data speeds while on a call, and as long as you're connected to someone also using HD voice on AT&T, you'll have a very clear connection. I happened to discover this when doing further reading from today's joint announcement this by AT&T and Verizon that they will support VoLTE interoperability between the two carriers in 2015.
Both of the photos were taken from my home office just a couple miles from the AT&T world headquarters in Downtown Dallas, TX.
More info from AT&T on HD Voice: http://landon.co/10rKqBw
More on the joint announcement from MacRumors:
UPDATE: 11:09 PM I placed my first VoLTE to VoLTE call with HD Voice on AT&T's network (The other calls were VoLTE to 4G or POTS.) and the quality is noticeably better. Definitely more along the lines of Skype or FaceTime Audio calls.