How does a firewall on your home network stop me from accessing any site on my cell phone if I visit you?
No matter what they do to try and stop phones from being smuggled in some will get in. There will always be some that get through. There are a large number of people that work for a prison and someone will have a bad time or be corruptible. With proper procedures and double checks you can get this down to a small number but there will always be a small amount of contraband getting through.
Changing the firewall settings won't do anything because they are on a different network. The need to suppress the cell network inside the prison and only within the prison. Guards, other employees, and visitors won't like it.
Unfortunately it's a symptom of having only enough money put into the system to house and punish those found guilty and not rehabilitate them. We keep them completely shut out of society with no preparation on how to re-integrate and then just shove them out the door with a few dollars in their pocket. Can you imagine trying to catch up on all of the changes in society if you have been away for a decade or two? I'm not suggesting that they don't deserve jail time but I think we need to do a way better job in our treatment of convicts.