I currently have the ATT unlimited charter plan, so the SIM card is ATT. I am evaluating changing to Cingular SIM and getting a different plan but have mixed signals from salespeople. One says the ATT plan is the best on the market, but I have to get an ATT phone. The other says that I need a Cingular plan because I can access many more towers and will therefore have less dropoffs. What is the truth, and what is the best plan and phone for my money? I don't want to pay roaming charges or be worried about minutes(I am spoiled at $99 plus tax a month). Also Bluetooth that might work with a GPS system might be cool. But in a convertible? Sorry, lots of questions from a non-techy.