if your old sim card is a old AT&T that i believe is white with the att symbol then you are on the old AT&T network. Yes the old at&t and cingular do use gsm sim and phones but they are two different networks. so what you would have to do is buy a unlocked phone to work with you old att card or what you would have to do is get a new plan with cingular. now if you keep waiting your plan will be release in a few month. I believe i heard it would be like around feb of next year. so if you dont change over to the new att network you phone will be turn off. you should of received some letters in the mail for this.

so if you still want to use the old att sim you need to purchase a unlocked phone cause no one manufactures the old att network phones anymore. sorry

I have an old Nokia 3120 GSM with a valid SIM (about 3 years old). I wanted to keep my old plan so I bought an LG CU400 on EBAY. The CU400 came without a SIM card. When I transferred the SIM card, the CU400 said my SIM from my Nokia is invalid. The Nokia sim is for ATT/Cingular and the new phone has Cingular written on the face of the phone. I thought all GSM SIM cards were compatible. Do I have to unlock the phone?
The SIM card fit into the CU400 perfectly, that is, it physically fit. Do I need a newer SIM card or just unlock the phone? If I need a newer SIM, how do I get it working to my old plan? I am getting the impression that I need a 64K sim that works with 3G. Is this correct? If I buy a new 64K SIM on Ebay, will Cingular activate it or will they force me to sign a new contract? Is my old SIM useless with newer phone that has 3G? How can I retain my old plan?

HELP!