Have you considered that the drivers phone number is used just for his flexible side gig (ie driving for "Uber")? They might not check that phone again for many days.
Its also possible that another passenger found your phone and said nothing to the driver, and took it. After all, cheap rides on the platform mean the driver does not have time between trips to clean the car and check for lost items after each trip. So even if they heard your message, they may have ignored it because there is no phone to be found. Its also possible that you did not leave a very clear phone number in your voice message to enable the driver to return your call.
Finally, the driver is an independent contractor. Once the trip is finished, there is no further contractual relationship between you and the driver. This is the business model that "Uber" set up. Drivers are not employees and may not also drive exclusively for "Uber." Your issue is now 100% between the provider you booked the trip with and you. In turn they can contact the driver.
I can speak from past experience in saying many passengers with lost property do not treat drivers with the respect and dignity they deserve (you may well have). It is not uncommon for passengers that did not take responsibility for their own property to expect a driver to drop everything they are doing, and drive many km's, forgoing lost revenue or personal time, at their own expense. In return they are met with a "thanks mate, I'll give you 5 stars response." The provider you booked the trip with does not have a robust policy in place to return your item with the urgency you would expect.
If you left your phone in a bank on Friday, would you expect a bank employee to go into the branch on a long weekend, recover your phone and deliver it to you at no cost to you on a public holiday? Of course not. You need to wait for "Uber" to follow up with your message. You chose Uber to convey you on your trip, not some guy in a Camry. You are "Ubers" customer, not the drivers..... you need to experience their "customer service" to get this resolved.....and you may find that experience underwhelming.