The 503 Service Unavailable error can indicate a few things. Usually, it means either your service provider is not able to fulfil your registration request, or VoIP phone was unable to route its registration to your provider.
We recommend first contacting your VoIP service 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 into the VoIP phone. Some VoIP providers are not compatible with a specific type of lookup that some VoIP phones will try to perform, but specifying a port and transport type should fix this.