This has been a very frustrating week with the same issue, but if your carrier is T-Mobile I finally found the solution with the help of an employee. The T-Mobile Name ID app does not work with Android Pie - it incorrectly flags all calls as likely scam and they get sent to voicemail almost immediately. The solution listed in the T-Mobile employee knowledge base says to upgrade the Name ID app, but there was no available upgrade so we uninstalled it and immediately the phone could receive calls again.
So, if you use T-Mobile, uninstall the T-Mobile Name ID app. If you are on another carrier, perhaps check if you have an app that rejects scam calls and try removing/disabling it.