Inmates have a variety of ways to hide cell phones but the most common place and way that an inmate or prisoner hides a cell phone is in hard covered books that have a section cut out of the book to fit the cell phone in.
The way inmates or prisoners usually get cell phones inside the the prison to them is through the staff as some staff will becomes friends with the inmates and will smuggle a cell phone into the prison for them and pass the cell phone to the inmate who then hides it.
Some visitors may smuggle the cell phone in through their panties or underwear or may even hide the cell phone in their shoes etc.
Then when they are visiting the inmate they will pass the phone to them.
But usually cell phones are passed to the inmate through staff because staff do not have to go through as much screening as visitors do.