hm, Why do you worry about privacy on a radio--that is, a cell phone? The stuff is transmitted in the clear for the most part. It doesn't take much to receive cellular transmissions, many software defined radios will do this for you. If you need privacy for a conversation (and my feeling is that if you want it, you should have it) then you need to take steps to guarantee your own encryption.
Illegal cell phone detector needs a receiver of all the bands of operating frequencies, a decoder,and data storage system. The software is to be so advanced that it can decode any calls received and transmitted.Quite a specific application.