New install of Ring Pro. App installed on two phones, one IOS 12, the other IOS 13. On IOS 13 Apple is more restrictive about what features an app can access. On both phones the Ring App asks if you want to enable notifications (necessary to get alerts from the doorbell). But it doesn’t ask if you want to enable the App to access the microphone. IOS 12 doesn’t seem to care and so the person at the door can hear you. But on IOS 13 the default is for the App not to have access to the microphone. The result is that on an IOS 13 iPhone the person at the door can’t hear you unless you go to Settings and specifically give the Ring App access to the microphone.
On IOS 13 the Ring App should ask if it can have access to the microphone when you install it, and it should note that if you don’t give that permission the person at the door won’t be able to hear you.