Phones can also transmit beacons. The same as laptops, actually, some of them constantly broadcast BLE beacons.

The signature is based on physical layer characteristics, according to the linked paper : « They result in two measurable metrics in BLE and WiFi transmissions: Carrier Frequency Offset (CFO) and I/Q imperfections, specifically: I/Q offset and I/Q imbalance. ».

Yes, you would probably need SDR to do this tracking, but SDR are generally not this expensive.