This is not (and will not) be possible.

Calling and the cell network is handled by the 'phone' itself and there is only a basic API to call phone functions from Maemo. The phone adjusts automatically to local surroundings, power transmission, which cell to use, etc. Being a Nokia it should be good at this, as they are traditionally strong in this area. From my experience the N900 seems to be good, but I think most of the other guys have caught up with Nokia in this department now anyway.