The day of your departure, one hour before the time you are supposed to drop-off your car, you will receive a text message with the contact details of your valet in order to call him 15 min before you get to the airport/train station. On your return, you will receive a text message with the contact details of the valet 2 hours before you arrive at the airport/train station in order to contact him as soon as you get your luggages back.
For more details, refer to the section How does it work? .