If it's a stock, unlocked (or ATT branded) C2-01 with the standard UMTS frequency bands for North America (850/1900), you should have had no issue connecting to a properly configured ATT cellular site with functioning 3G service in 99.9% of the United States.
You state you tried working SIM cards/phones on other working cellular sites (correct terminology vs 'towers'). This means either the phone settings were incorrect, or they botched your account provisioning. It happens. And CC is probably one of the worst for CS too. Horrid.
Very soon you will need to upgrade the type of phone you prefer for daily use to make your life easier. Otherwise, you hope what you find on fleabay or amazon/overstock.com is going to work properly. Good luck.
I am looking for a simple GSM bar phone (aka feature phone) that works on a 3G network. My provider is Consumer Cellular--they use the AT&T network. 2G is being phased out, and recently the 2G tower stopped working in my area. I bought a Nokia C2-01 bar phone because it advertised that it was 3G, however, it will not work on the new towers. (This is after hours of trouble-shooting: new SIM cards [including a genuine AT&T SIM card], several Nokia phones, and different locations around the city.) The phone will connect with a 2G tower, but not with a 2G-3G tower. Nokia phones were sold to Microsoft a while back, and all Microsoft is concerned with is their Windows phones, so they don't provide any support for simple Nokia phones. I don't want or need a smartphone, and don't even want or need a digital screen. Is there still a simple push-button phone available anymore?