The way that prisoners would charge up the cell phones they smuggle into the prison would be with the phone charger that is also smuggled in and they just plug the cell phone charger into the electrical outlet in their prison cell.
Most prison cells do have electrical outlets in them from what I've seen on TV.
I would assume that most prison cells do have electrical outlets because some prisoners have televisions and even electric fans to keep them cool.
So in that case the prison cells would have electrical outlets where the prison inmate can charge the cell phones.
Now as for supermax prison cells or Solitary Confinement prison cells I would think that those prison cells would not have any electrical outlets in them.
But the prison cells I have seen on TV had electrical outlets.