S80 Onboard TDMA Phone

[1]

I have a 2004 S80 T6 with onboard TDMA phone that works with AT&T wireless.
It is a nice feature, however, it is not compatible with my AT&T GSM account
(I use an AudioVox 5600 on this account). I would like to upgrade the
onboard to GSM and want to know if anyone has info if this can be done???