Ha, very nice one!

First determine if GSM is a necesscity to control. 
If yes: good luck. Is Qualcomm in the deal? Nice if yes.
If no: possible.

Then go back to NIP-01 and the Nokia 3310. Contacts are npubs you saved, nothing else.

I‘ve got a Firefox phone lying around for which I wrote apps in JS.  I‘d start there.

Alternative:
Create a proprietary iPhone and Android app called NIP-01 or „NIP Phone“ and do everything on the existing infrastructure.

Considerations:
Are LoraWAN and HAM networks a valid entry point to the internet? Mesh-routed calls? Complicated.