Re: Ready for the down votes, but...
"a few low powered jammers dotted around the prison buildings would suffice."
Based on my limited knowledge and experience, the answer is no, that will not suffice.
NZ Corrections has some barmy contract for some private company to block mobile signals. Original budget was 20 million plus a million a year to run it. Currently cost ~2 billion, and is about 95% effective (at the cost of ruining mobile around prison sites), meaning that almost all confiscated phones fall into the 5% of phone+sim combinations that work. There are also often spots where you can get a clear signal which are not apparent in a site survey, but are to lags who have a lot of time on their hands.
Having the telecoms company trace and log/block phones was the more sensible and realistic option, but wasn't done as it would have had a higher running cost.