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 sip.virtualglobalphone.com with the 151.106.10.6 might help. 

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.

Or simply restart the phone or your Internet service provider. 

For more refer : https://www.zoiper.com/en/support/home/article/61/SIP%20503#android