Hi James, have you already tried to uninstall/reinstall your app?
Hi I assume you're using the Mobile App and not the desktop one.
If you are using an iPhone, I would start by verifying that microphone access hasn't been revoked. Go to Settings, scroll to and tap RingCentral, and verify that the microphone slider is set to "On".
Next I would try changing the calling mode from within the app. Open the ring Central app, tap your profile in the top left, and then tap phone. Under calling mode, try switching from direct dial to RingOut.
Lastly I would try changing the direct dial data to wifi or carrier minutes. This setting is right below the previous.
The Quality of service tab in the Ring Central Analytics portal may give you some insight as well.
Best of luck!