Paigntonian has largely answered your question.
Many prisoners who try to get hold of phones in prisons though simply want to increase contact with their families and kids. They only have limited means to earn the money to pay for calls from prison payphones and access to those phones is also very limited. (If the only time that a prisoner can use a phone to speak to his family is when his kids are at school, he's likely to want to try to find another way to contact them).
So, in a perfect world, it would make sense to allow all prisoners to have mobile phones in their cells. However the world isn't perfect and there are a lot of people in prison who want to arrange for drugs to be brought in, for witnesses in forthcoming court cases to be intimidated and to continue running the county lines networks that they'd masterminded before they were sent to prison. So there's a blanket ban on mobile phones in prisons, meaning that those prisoners who simply want more contact with their families lose out.