In Mexico, where I live, inmates having cellphones is a huge problem because they use it to commit more crimes. Not only do they do their old drug-related deals via phone from the "safety" of jail, but they actually got creative a few years ago.
You will receive a phone call and they'll tell you that it's Captain Gonzalez (or whatever) from the Federal Police. They caught your niece (sometimes they even tell you the name of your niece, sometimes they just say niece) with drugs and unless you are willing to pay the Captain 5k pesos to let it go, she'll wind up in jail for years. Or they tell you they have your cousin, that was trying to cross the border into the US illegally and they'll have him jailed unless you pay up.
Sometimes they wouldn't even pretend to be anything and just be straightforward about it: "We know you live at ADDRESS and your wife is NAME and you have 2 daughters who attend SCHOOL from SCHEDULE. If you don't deposit 10k pesos in my account, you will regret it".
Most people payed up out of fear. It's easy enough to prove that your cousin or niece is free and about, but many people didn't do it. They just paid. My grandmather was victim of such a call, twoce. She didn't actually pay because my mother told her it was a scam, but that didn't remove the fear out of my grandma and she doesn't pick up the phone anymore.
It took only a short while for police to realize that most of these calls (which were really common up until a few months ago, probably because people now know about them and just ignore them) came from prisons, with smuggled cell phones.
Absurd measures were taken but of course they didn't work. Now the calls have ceased, I believe, but just because people's awareness of the issue.
So yeah, inmates having cellphones IS dangerous and should be avoided at all costs.