Firewall is blocking media stream?

My shiny new Air refuses to connect to VOIP dialers, or to FB Messenger video calls. It keeps telling me that "Your firewall is blocking media stream. Please contact your system administrator or use alternative method to dial in", or it'll just pretend that there's no camera or mic. This happens in Chrome, Firefox and in Safari.

Thing is, I don't have the Mac OS firewall turned on.

"Firewall: Off

This computer's firewall is currently turned off. All incoming connections to this computer are allowed."

The router firewall is set to

"Typical Security (Recommended)

Inbound Policy: Reject.

Remote Administration settings will override the inbound security policy.

Outbound Policy: Accept."

Zoom calls work, most of the time. But VOIP and FB Messenger video calls? Nothing. All the settings claim that camera and mic are allowed, but they don't do anything.

So, what do?