I think prisons have fixed lines, just a guess. You know the world did function prior to the introduction of the mobile phone. And you're about one step from "Think of the children!"
I don't know what the figures are now, but in August 2004, 29% of emergency calls to ambulances in central London were by mobile phone (http://publishing.rcseng.ac.uk/doi/pdf/10.1308/003588408X242079).
And the odds of dying at the scene when the incident is called in by mobile phone are only 77% of the odds of dying at the scene if the call is made on a landline (https://www.ncbi.nlm.nih.gov/pubmed/22142669). Even if the world did function prior to the introduction of the mobile phone, and indeed the steam engine and the wheel, mobile phones are now critical life saving aids and widely restricting their ability to call the emergency services would certainly result in many needless deaths.