Maybe it has something to do with the security of a phone, I remember President Trump saying he doesn't like cell phones, and on AF1 cell phones may not work as well and would NOT be secure. I did some research and found this, "The Airborne Executive Phone These new phones aboard Air Force One can be recognized as the Airborne Executive Phone (AEP) made by L-3 Communications. This is a military contractor that, among many other things, also manufactures the STE, the secure desktop telephone that is most widely used by US military and government. The Airborne Executive Phone is able to make both secure and non-secure calls from a single handset. It also provides Multiple Independent Levels of Security (MILS) for digital voice and internet data access. This should provide end users with the experience of "reliable connectivity, interoperability and security they would have in an executive office environment".