I also have Att, but so far 4 guys have complained that they cant hear when i called, called second time it seemed to work, but they kept saying my voice is breaking off, never had this issue with my 6s though
You might want to check the box and make sure that you got the SIM FREE version of the iPhone.
The AT&T 7 Plus version of the iPhone is A1784. I have all kinds of trouble with that iPhone in areas of marginal signal quality.