This error may have few different meanings depending on the entire error message.
Typically it 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)
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.
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.
It may also indicate firewall blocking 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.
Windows, Mac and Linux:
This error message might indicate that you have probably selected wrong transport type (UDP/TCP/TLS), or that there is no connectivity with the server. For example due to wrong credentials and the pbx is blocking you or something else.
Try changing the transport type, or consulting with the PBX administrator. Ask them to check if your IP address is not being blocked.
You can change the transport for the SIP account from Zoiper -> Settings -> Accounts -> your SIP account -> Advanced -> Transport.
- Transport failure
From the Network Settings in Zoiper, check if you are using the correct "Transport type" (TCP, UDP or TLS depending on your VoIP service provider requirements).
- DNS Timeout
The DNS server fails to resolve the domain of your VoIP service provider in a timely manner or completely.
Rebooting both your computer and router may help.
If you use any third party custom DNS servers, try removing them. Or if you use the one from your ISP, try using a public one like the one from Google.
In some cases this error message may indicate that your VoIP service provider has blocked you or you are using bad/ wrong domain name, you would need to clarify this with them.
You can try using the IP address of your service provider instead of their domain name. You should also clarify with your VoIP service provider if you are using the correct hostname for your SIP service.