Joachim,
thanks for your interest.
Looking at the native Contacts I've confirmed thst there are only entries in the Google account.
If I "click" Contacts to Display/Customise/<MyGoogleAcct>/All Other Contacts then I see the full list.
if I "click" Contacts to Display/All Contacts I only see one entry.
So it looks like my problem is with Contacts in general, not with Zoiper.