Jammer is unneccessary.
The law doesn't have to be ammended. Make one room on the way into the prison a farraday cage. Watch for (RF) noise from phones as people *leave* the farraday cage -- the phone will reacquire the cellular signal as the user leaves the farraday cage, and blast out RF as it re-registers itself with the phone network. Detect this and take the phone.
If these jammers could be PROPERLY deployed (they can maybe -- some movie theaters etc. will apparently install them on the sly, since assholes INSIST they "have" to leave their phone on because they're on call, rather than realizing "on call=don't go to the theater"...or they just leave it on because they're a big asshole) then there'd be no problem. But something the size of a prison? Any jammer is going to leak, and the prison SHOULD pay fines if their neighbors start having problems. Prisons are large, it'll need MANY low-powered jammers to be effective, a single higher-powered one would certainly leak too much (less of a problem if this is an isolated prison, but many are right in town...) The cell companies spent $billions on spectrum, and have been *required* to build out either based on % population covered or % terrain covered. Obviously, the FCC has to be strict, or this spectrum will become polluted and useless. Realize for UMTA/WCDMA and CDMA systems, they are interference-limited rather than having hard capacity like GSM systems, these jammers could lower the capacity of neighboring sites for a large area, requiring the cell cos to spend more money on additional sites that they wouldn't have to if their spectrum wasn't polluted.
"The correct response is for guards to sue the CTIA next time a prison worker gets hurt and an illicit cell phone contributed to the prisoner's ability to do whatever bad they did."
A) That's stupid, noone's going to plan shivving some guard via cell phone.
B) If a guard got hurt, probably the first instinct if they can't get to an alarm button is.... to pull out the cell phone and call for help. I seriously bet the guards have cells on them.