Any one know much about GSM wireless networks? I want to buy a GSM phone that Cingular does not current sell, however it works on the same bans as ( 900/1800/1900) so it should work. Is all I have to do is put in my SIM card? I know they are not like other phones where you have to give them The ESN for the phone to work, however when I've been in to upgrade my phone in the past, they always seem to take a # of the new phones box and put it in the computer... Does this have anything to do with how calls are sen/t received from the phone? Any input is appreciated.