Used Phone

The used NEC N620 I bought in China doesn't seem to work with my other SIM cards. In addition to the China Mobile SIM I bought there, I have a Cingular/AT&T and a T-Mobile SIM. I remember both of them working in China when I inserted them into my China phone but all I get is "No net" here. The NEC is supposed to be a dual-band GSM (900/1900) phone while the Treo 680 from work and my RAZR V3 are quad-band GSM (850/900/1800/1900). I thought both AT&T and T-Mobile used GSM1900 in California but I guess they're using one of the two frequencies not found on my NEC phone. I can still use it as a Chinese-English dictionary and it has a cool majong game on it.