Call quality I tested the Windows Phone 8X in San Francisco using AT&T's network (GSM 850/900/1800/1900 MHz; LTE 1700/2100). Call quality was very good. Volume was nice and high at a medium level, with no noise or interruptions. My caller's voice sounded warm, rich, and fairly natural. There was a little flaw when he spoke, making him sound slightly lispy and thick, but we were able to talk for a long time without the device getting in the way.
On his end, my testing partner called the phone very premium, from a calling perspective (he didn't know which one I was using until the end). When analyzing the call, he said I sounded slightly unnatural, but loud, clear, noise-free, and excellent.