Please make sure you have the correct transport setting in Zoiper according to your provider's instructions. You might need to use UDP, TCP or TLS.
If using a SIP account, disabling STUN and / or Rport (or enabling it, depending on its current state) could help.
Please open settings -> Preferences;
click on Accounts;
select your account from the list on the left;
click on Advanced;
enable or disable STUN;
save the changes and try to recreate the issue.
Please also disable STUN globally for the application by following these steps:
- open settings -> preferences -> advanced -> network sub-tab -> STUN options