Hi Senthil,I cannot speak for AT&T in the U.S., but Rogers in Canada did have to change my network settings to provide a "data capability" before the Nokia 3360 modem could be used to connect to my ISP. From other postings, it would seem that AT&T California representative that you spoke to is either unaware of the Nokia 3360 capabilities or (perhaps?)is trying to switch you over from their TDMA service to their new GSM service. I also received the "no carrier" message until Rogers activated the data capability on my service account.
Incidently, www.nokiausa.com does mention the modem capacity of the 3360 and provides software (the PCSuite for the 3360) which enables quick and easy connections with Windows PCs, Palm devices, and the PocketPC machines. Alas, there was no support for EPOC machines so it was trial and error to get the settings for my Mako.
Good luck,
Lawrence Fast