A 503 Service Unavailable error can indicate a few things. Usually it means either your service provider is not able to fulfill your registration request, or Bria was unable to route its registration to your provider.
We recommend first contacting your VoIP provider to see if they are experiencing any service outages. If they are not, please ask for the port and transport type that should be used when connecting to their services, then enter these in Bria. Some VoIP providers are not compatible with a specific type of lookup that Bria will try to perform, but specifying a port and transport type should avoid this.
To do so, log in to the Bria Admin portal, go to the Voice & Video tab, and edit your Voice Server Configuration. Configure the port your provider specified in the corresponding field. You can then specify the transport to be used on the Service Settings tab of your Voice Configuration.
If this does not help please contact our support teams for further assistance