@andrewfeeney @L0G1S I dunno about the approach @SwiftOnSecurity describes.

But the IMSI catchers are "affordable" these days.

https://arstechnica.com/information-technology/2015/10/low-cost-imsi-catcher-for-4glte-networks-track-phones-precise-locations/

With an IMSI catcher you can also snitch up SMSes. And the SS7 signalling system typically used between telco companies to enable communication between phone subscribers regardless of the network they are connected to is riddled with security issues.

Basically consider all phone network communication to be broken by default. And make use of proper #E2EE on top of that network instead. E2EE gives you a protection which does not depend on your communication channel itself to be secure.