How do I build my own VoIP server with a SIM card to use when there?
Skipping over the how to I make my own cell phone sort of idea Im not sure what you are asking here Not at all. If there is no cell coverage, you need to use Wi-Fi to connect a SIP softphone app to its server, probably a VOIP PBX of some sort. But I am not sure this is what you want. Understand, the SIP softphone is going to have the phone number of the VOIP PBX, not the cellular phone. How do you get a cell phone to work without coverage? Simple, turn on Wi-Fi calling. If your phone doesnt support it, get a different phone. If your carrier doesnt support it, get a different carrier.