Well they could have been testing to se if the phone would work. I am sure ne of the ROPs that work at the airport placed the phone there and one of his accomplishes was on the flight to listen if the phone rang. Nest time an IED will be attached to the phone.
You and I may have equally warped minds -- hopefully. Both the number of Arabs and South Asians working at airports and the difficulty of effective screening when what you are checking for is essentially private religious belief and fervor, not criminality or poor work habits which are likely documented somewhere, many bad actors must be in the plane cleaner, luggage handler, electrician, mechanic and related ranks. This does sound like a test run.