[GadgetKen]

A couple of other suggestions:

1) You may want to pick up an old 2g or 2.5g AT&T or Cingular sim chip from a dealer or from an ebay merchant and see if it works better. If not, just use up the airtime and toss.

Why:
The firmware/hardware of mobile phones made primarily for the Asian market (with more limited overseas exports) tend to work better with 2G (GPRS) or 2.5G (EDGE) sim chips. Putting in a 3G (HSPDA) or 4G (LTE) sim chip *may* work in some handsets, but performance may be erratic. That's despite newer sim chips supposedly being backwards compatible. Explanations of different standards are here: GSM - Wikipedia, the free encyclopedia

Have no problems with my old 2G Beyond GSM/Jolt/AT&T sim chip, my Tuyo/T-Mobile sim chip or my Digicel Bermuda sim chips in the USA (the last two I think are 2g or 2.5g). But the 3G dual IMSI ekit simplecalling sim chip is tricky in my two multi-sim chip Chinese phones. It will work in the first two sim slots of my quad sim phone, but causes phantom phone reboots in the last two slots. So I always keep the ekit sim chip in one of the first 2 slots.

2) When can't place a call with AT&T, try turning the phone off and turning it back on again(if all else fails, pull the battery as well). That may re-authenticate the sim chip on the local mobile telephone switching office. Also another trick is to dial *777# for a text message balance check so AT&T has to look up your account, that sometimes will also set things right. Or do a network search, select at&t and attempt a forced registration. But just turning the phone off and on seems to work the best.