The issue that you are experiencing might be because the DNS server does not reply at times. (this could be a firewall issue, a problem with Windows or a problem with the DNS server)
Replacing the hostname of your VoIP provider with the server IP address might help. If you do not know the IP address of the server, contact your VoIP provider, explain the issue and ask for the IP address of the server.
SIP 503 error message might be also generated when the service you are trying to use is unavailable.
Make sure that all account details and the server hostname are entered correctly. If the issue is appearing randomly it may be temporary issue with your VoIP provider.
For additional information and assistance regarding this error message, please contact your VoIP service provider.
If you are using TLS as transport type, you might see this error message if there is a problem with your certificate file.
Android:
Android 6:
Error 503 sometimes happens when the Android firewall suddenly decides to block access to the DNS server.
The Zoiper log file will have a line like this:
Transport.cxx:199 | Some other error (1): Operation not permitted
As a workaround, try the Settings screen and select Data usage. You can tap a specific application and check the "Restrict mobile data" check box to prevent the application from using mobile data in the background.
No route to destination
This error usually means the network is not active yet. Try if you can use a browser to search for something on Google. Most likely that will not work either.
Try to fix the network first, then try to re-register in Zoiper.
No transports left to try
Android:
In rare cases this might occur after a network change. The issue seems that Android signals a network change before the network is completely up and as a result the DNS lookups fail. We are trying to work around this android specific race condition.
When you see this error on Android, please contact us on support@zoiper.com so that we can help to troubleshoot the issue.
Transport failure
When you see this error on Android, please contact us on support@zoiper.com so that we can help to troubleshoot the issue
DNS Timeout
When you see this error on Android, please contact us on support@zoiper.com so that we can help to troubleshoot the issue.