I'd imagine prison authorities wouldn't be too happy about visitors wondering into the governor's office
We used to have these things called telephone boxes which were very handy - try to find a working one now.
I actually had this very problem in Crowborough: I needed to order a cab but...
It would also not distinguish between prisoners and staff/visitors. For example, I imagine that quite a few visitors would need to order a taxi - how would they be able to do that if mobiles are blocked?