I wonder if there's an argument that mobile phones are using publicly-owned broadcast spectrum, and therefore, like when walking around in public, there isn't an expectation of privacy. Ham radio rules for example prohibit encryption because they are using public airwaves.
It used to be possible to just set up an analog receiver and you could listen to nearby cell phone calls.
I actually find that a decent argument, but I believe it’s already been shut down. When police used thermal imaging to “observe” a house, without doing anything they thought required a warrant, the Supreme Court decided it was too intrusive ( https://supreme.justia.com/cases/federal/us/533/27/#tab-opin... ). But if I understand the ruling correctly, if the police make it well-known that they have the ability to do something intrusive, then they don’t need a warrant.
Intercepting cell phone communications is restricted by law. This was enacted when public officials had their clear AMPS phone calls recorded with legally owned passive radios.