Have the same problem here.
I am using "sipgate DE" as provider and have this issue.
Fix available WHEN please?
We have a fix ready that will now go through to QA testing as we don't want it to have sideffects for other customers, if all goes fine tomorrow this will be uploaded to Apple for approval. They will need some time for review as well.
just to be clear: this is a bug in Zoiper that was not caught in QA as it seems to be triggered only with this specific setup and did not happen in the lab or with the beta testers. The provider is not to blame.