I hope you are prepared for the legal reprocusions of blocking an essential service. If you block an important call for a police officer, emergency service worker, or other important service operator, you will be legaly responsible. If there is a loss of life involved, you will become an accessory to the fact.
On the other hand, mobile phones are very sophisticated, and are controled by the systems that they are subscibed to. There are many frequencies covered. In any case, you would have no way to control the frequency output of the phone without getting in to some very sophisticated software design. The bandwidth of coverage for these phones are vast.
One thing for sure, if you do find a way to make a celiar phone jammer and get it working, make sure you have a very good attourny, because you will need one when you get caught. It is a serious federal offence, even on private property.
Jerry G,