I am on cingular, but I have no problems with GPRS dial-up, on a Sony Ericsson T-616. I have the latest firmware...etc.

I guess technically I have heard that Cingular doesn't want you using your phone as a modem, but what the hey, I pay extra for the data plan, and I don't want to use any of their lame palm pdas.

I don't know... how are other people's experiences with T series SE phones?