“...in the US legal.”
No it is not.
Among other laws there is the “Electronic Communications Privacy Act” which forbids intercepting cell phone calls. That law even made it illegal to sell radios (scanners) that could receive a specific band of frequencies, which were the cell phone frequencies used at the time the law was passed.
Yes, but do you think the NSA gives a shite? LOL.
They have been eaves dropping on private citizen comms ever since Obama was inaugurated.