As long as it’s switched on, every cellular device connects to the nearest cell site from time to time, so the cellular companies know where to route received messages.
Obviously, when receiving a text message the transmitter must come on to establish the connection and provide error checking.
That said, I’m sure the Secret Service would never allow Pressidential communications devices to connect directly to the cellular system. Wherever he might be, to the cell system, the Presidents communications would allways be seen as coming from and going to, wherever they have their main gateway located.
Ok. But we were GPS, then ‘electronic noise’. The agents around Zero better not have their cells on then...