Try this
Yes, phones even in the receive mode put out transmitted signals to keep the node points aware of their existence in their area. This way when the phone goes to the next geographic zone that one knows to handle any calls.
Use the old military EW radar warning receiver method. Go antenna- wide band amplifier covering each band-detector-video analyzer.
The pulses out of the detector will be set by the phone protocol so you need to do some complex analysis which a uP can probable do. Pulse rep rate and width will probably be enough for each protocol on each band.
The hard part is the ones that are on all the time like spread code multiplexed. You can do a level detect as long as your system does not respond to out of band signals. I suspect that some simple band pass filters would help. Also having several such units at different places in your geographic area (meeting room, hospital, or whatever it is) will increase the probability of detection.