Conversely, when attempting to initiate a call via the RingCentral app on my mobile phone, once I dial the number and press the green button, the next screen shows a "Connecting...." message for a few seconds and then a popup appears about the app unable to connect to the network (hard to get the entire message since the popup only stays on screen for a second or two).
All other parts of the RingCentral app seem to work normally.
Have uninstalled, rebooted, and re-installed the app with not change in behavior.
Phone is a OnePlus 6T, running the international version of the firmware on the T-Mobile network.
Have not yet attempted to make or receive calls while the phone is connected to WiFi.