The maximum penalty for smuggling unauthorised items into a prison is 10 years.
Some prisons would be easy, such as an open one, which have many phones in them already.
Others, such as local prisons are more difficult, and maximum security is very difficult, but not impossible.
HOWEVER, remember that if you attempt to take one in and are caught, you will be arrested, and may end up in prison yourself. Is it worth it?
Mobile phones often turn up inside a prison - they are caught on spot checks or intelligence. There will then be an adjudication hearing and the likely outcome (if proved) is time added onto your sentence.
Going back to the original question. I once answered a cell bell to a prisoner who was on his mobile to his wife, as she was at the gate and they wouldn't let her in.
As I was standing there, he was talking to his wife on her mobile!
Then he realised what he was doing, and handed it over to me.
Bless him.