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 open your Softphone menu -> Account Settings -> Choose your SIP/VoIP account. Affix the port your provider specified to the end of your domain. If you were connecting to sip.example.com, and your provider specified that port 5060 should be used, your domain category should then be sip.example.com:5060. You can then specify the transport to be used on the Transport tab of your account settings.
If this does not help please contact our support teams for further assistance