Uber drivers are unreachable

Can someone please tell me how to speak to Someone from Uber. Yesterday, my son forgot his phone in the car whilst riding back from school with a gentleman named Vitumbiko Mkeya, registration KZ80NL GP, a grey Hyundai i10. I immediately tried reaching him and he dropped my calls. I selected the option for the message to be relayed to him and have still not been successful reaching him nor have I heard back from him. I have exhausted every avenue. The fact that everything should be done via the app that yields no results is completely dissappointing and frustrating. I need someome to take accountability.