Actually, no. Phones don’t always broadcast their IMSI. Most of the time they broadcast a Temporary Mobile Subscriber Identity (TMSI) and only on a location update (For power conservation). Your cell provider knows your IMSI already and uses a TMSI for updates for the express purpose of privacy and security for these exact scenarios
It is part of the initial work flow of a Stingray device to attempt to force your device to disconnect from the network and get it to rebroadcast its actual IMSI. But it is not floating around in the air all the time and it certainly isn’t trivial to grab.
BT is really the only viable option, and even that can vary wildly depending on manufacturer.
He’s got my vote lol