whenever this topic comes up, I like to point people to EFF's Rayhunter.
https://github.com/EFForg/rayhunter
> Rayhunter is a project for detecting IMSI catchers, also known as cell-site simulators or stingrays. It was first designed to run on a cheap mobile hotspot called the Orbic RC400L, but thanks to community efforts, it can support some other devices as well.
And
> bitchat is a decentralized peer-to-peer messaging application that operates over bluetooth mesh networks. no internet required, no servers, no phone numbers.