Some Apple iPhone users on AT&T can't make or take calls (UPDATE)
UPDATE: The outage now appears to be affecting all AT&T customers regardless of the device they use.
AT&T's @ATT Twitter account tweeted an easy solution that seems to work. Restarting the affected iPhone (multiple times if necessary) is supposed to help iPhone users reconnect to AT&T's network, thus allowing them to once again make and take phone calls. One person says that while this tip didn't help her, she went to airplane mode and turned it on and off three times. It might sound strange, but it worked for her. If restarting the phone or playing around with airplane mode doesn't allow you to make or take calls, you can try FaceTime, or FaceTime audio. And you can also use iMessage to communicate.
The problem began at 3:40pm EST and peaked at around 5pm EST. Website Down Detector says that current areas affected include Houston, Mountain View, Chicago, San Francisco, Austin, New York, Los Angeles, Dallas, Orlando, and Arlington. Overall, more than 11,000 complaints were tallied by Outage Reports today, including more than 700 in the last twenty minutes.
source: @ATT, downdetector, OutageReport