Due to happy coincidence, the existing NetworkManager mobile broadband support isn’t just limited to mobile broadband cards. You can most likely use your GSM mobile phone with NetworkManager (and the VPN too!) if you add the appropriate stuff to the HAL .fdi file. Tag the serial port the phone exposes like so:
<!-- Nokia N80 --> <match key="@info.parent:usb.vendor_id" int="0x0421"> <match key="@info.parent:usb.product_id" int="0x0445"> <match key="@info.parent:usb.interface.number" int="8"> <append key="info.capabilities" type="strlist">modem</append> <append key="modem.command_sets" type="strlist">GSM-07.07</append> <append key="modem.command_sets" type="strlist">GSM-07.05</append> </match> </match> </match>
and as long as you’ve got a data plan, it’ll probably Just Work.