Unless You Have A Very Eager Owl?

I’m working at a call center for a car-sharing company. Customers pick up and return cars to certain reserved parking spots, which occasionally get taken by other cars. The ensuing phone conversation happens at least once a day.

Customer: “There’s someone in my parking spot!”

Me: “Okay. In that case, you’ll have to find a different spot to park the car; just let us know where it is so we can call the next customer and inform them.”

Customer: “How do I let you know?”

Me: “Call us back?”

How else, by carrier pigeon?