How to stop cell phone signal?
Undoubtedly, the most convenient method is to use cell phone signal jammer. There are ways to build Faraday cages, but they are expensive and not portable.
Cell phones are essentially handheld radios, and the jammer's signal is strong enough to overload the cell phone and cut the call. If you were to look at the signal indicator on a phone as it's being jammed, you'd see it immediately power up to "full bars" and then quickly go to no signal. Once the jammer is off or the cell user leaves the radius, the phone works normally again.