Whilst they are hard to make perfect, there are certain companies who do indeed employ shielding that cuts out the majority of outside signals. You can still find certain parts of buildings where you get a signal, but for a prison it would probably be easy to cut out all the cells. Just leaves outside when they exercise/whatever, but surely easy enough to search them and also spot when they're on the phone.

But yes, it does cut out other radio signals too.

Would surely be cheaper though that a jamming signal, especially considering the prisoners could probably sue for harmful effects of such a signal (especially in America).