Chris Slater-Walker

UK

Network Engineer

Although I'm no expert in the technical details of GSM, I believe the following is correct:

Jamming mobiles is not that simple. In order to jam reliably, the jammer needs to jam every available channel across the whole frequency spectrum allocated to mobile phones; or if the intention is just to jam a single mobile, then the jammer needs to track that mobile as it changes channel regularly (at least I believe that's what happens).
Mobile base stations are often quite powerful but that power is spread across all the channels in use at any given time, and a single channel is capable of supporting a number of calls at once. Therefore a single-frequency jammer will have a limited effect at best.