I, too, called AT&T technical support.  Here are the steps they recommended that worked for me:

1.  Power down the iPhone.

2.  Remove the SIM card from the side.

3.  Power up the iPhone.  (under network, you'll see "Searching....")

4.  Slide the SIM card back into the phone.

5.  The network indicator will alternate between "Searching...." and "No Service" for about 30 seconds.

6.  When it indicates it's found AT&T as the network, the M-Cell tag should pop up as well.

So, we have this problem solved.  I hope this helps others.