I thought most such services would ask you to phone once you are ready to depart, or close to departing, the termnal (you can't use the phone in Immigration-- not sure about baggage and Customs), since they know the amount of time required can vary and they don't want to have their driver sitting out at the curb for long periods of time, possibly incurring a fine. The driver may circle the airport road or wait in a parking area. I'd ask the service to confirm.