I ran into the same headache trying to fax documents from my phone using just the sim card frequencies - turns out most cellular networks dropped analog fax support years ago because faxing requires a stable, uncompressed voice channel that modern VoLTE and 5G just don’t guarantee. What actually works is an online fax service that sends via the internet instead of trying to brute-force through a mobile network. I’ve been using https://mfax.to/ for a few months now: you upload a photo or PDF, pick a cover sheet, and it goes out as a real fax over regular phone lines on their end, with delivery confirmation and HIPAA compliance if you need health stuff. No special sim tricks required, just wifi or mobile data. The average send time is under four minutes, and the success rate is 99.9% - way better than the old “hold it to the speaker” method I tried first.

At least over the wifi with encryption....a Proton FAX for Android :P