When a phone is static it will 'ping' on just one mast, usually the nearest. When a phone moving it will 'ping' on a number of masts in succession, thus indicating is direction of movement, a tracking technique that often used in crime detection work. The phone company would therefor have been able to report that Simon's phone wasn't just 'near' the airport. They would know whether it travelled to the airport at that time.
It would be one of the first things that any investigator would want to know. "Where was he and what were his last known movements'.