Theoretically, both would be possible. The cell phone modem is a separate hardware piece in your phone, it has its own operating system. The smartphone OS comm

Theoretically, both would be possible. The cell phone modem is a separate hardware piece in your phone, it has its own operating system. The smartphone OS communicates with the modem over an interface usually using "AT" commands. The iPhone 1-4 had an cell phone modem design by Intel (GSM version, modem manufacturer got bought by Intel). The iOS UI consists app(s), so it may crash and restart.