The N810 does not have the hardware required to send or receive cellular signals. The N810 is not a phone. The only way you could make it a phone would be to open it up and somehow add the hardware (very complicated, if not impossible).

The only other way to make calls through it would be to use it as a VOIP phone, or a device that allows you to talk to other people over an internet connection. Google Voice + Gizmo/Skype makes for a good free VOIP phone. However, this method does not help if you have no wifi signal.