Again, you DO NOT need a cell phone tower to send and receive SMS and phone calls. You can register your phone on its home network near a tower and go WiFi-only forever after.
“ One issue is that the protocol for wifi calling is notoriously opaque. Carriers frequently change the underlying infrastructure and protocol details.
Also, the protocol assumes terrestrial broadband with reasonable latency and bandwidth. At McMurdo, as of this writing, latency to terrestrial locations is in excess of 700 milliseconds. Usable bandwidth for any given end user can vary widely, down to a few dozen kilobits per second.
The protocol also doesn’t expose any useful diagnostic info to the end user in order to troubleshoot. You just have to cross your fingers that the magic “wifi calling” icon lights up.”
This is how it works on paper. You may have had success with this.
It does not work universally. It won't take long to find a bank / FAANG / service provider that refuses to accept anything but a bona fide mobile SIM talking to a base station.
How does a SMS sender know this? Is there some mechanism in SMS to only deliver if by certain criteria?