They all use frequency hopping, in addition to encryption and such. This means that they automatically retune to a new frequency many times per second. A simple tuner can only stay on one frequency. Even if you have one that can hop, it needs to know the pattern to follow in order to track the signal, and the agreement on a pattern between the sender and receiver is kept secret by encryption. The common way to intercept cellphone communications is to have your own device that acts like a cellphone tower, trick the phone into connecting to that, and read off the data after it has been received, while transparently forwarding it to the real cellphone network.