These days it often happens (well with BA anyway) that you get boarded so that they can report "fully ready" to ATC hoping for an improved slot. It's only once you're aboard that the Captain informs you of this and if no earlier slot materialises you can be sitting at the gate for anything up to one hour. At Brum we even taxyed to the loop by runway 15 and shut down whilst awaiting our slot. Under these circumstances I can understand pax wishing to make a quick call with a revised ETA. Same happened in the USA when we had to hold off gate due to a groundhold imposed on traffic inbound to our destination. Engines were shut down and the Captain explained the problem and then went on to tell the pax that they could use their mobils until the engines were re-started.