What to do if you leave something in a car
Act in the first ten minutes and the odds are good. Act tomorrow and they are not.
Phones, bags, documents, an umbrella. Things get left in vehicles, and the difference between recovering one and not is almost entirely how quickly you act.
First: do not call an unknown number
Your instinct will be to phone the driver directly. In-app calling exists precisely so neither of you has the other's personal number, which means after the trip you do not have a number to call.
That is a privacy feature working as intended, not an obstacle. Use the right channel instead.
Contact support with the trip reference
Every trip has a reference, and your trip history has it. Report the lost item against that trip. This matters because it lets support reach the correct driver, on the correct trip, with a record of when and where.
A message that says "I left my phone in a Topsy yesterday" without a reference is very hard to act on. One with a trip reference is a specific request.
Be precise about the item and where it was
What it is, what it looks like, and where in the vehicle you think it was. Drivers carry many passengers; "a black bag" describes half of them.
Understand the driver's position
A driver who finds an item is often mid-shift and some distance away. Returning it costs them working time. Be reasonable about arranging the handover, and be aware that a driver going out of their way is doing you a real favour.
The prevention that actually works
Check the seat and the footwell before you close the door. It sounds trite. It is also the only reliable method, and the moment to do it is while the car is still there rather than after it has pulled away.
Phones slide out of pockets on car seats. If you sat with a phone in a back pocket, check the seat specifically.