Hi Pete,
Please try this to set zoiper as default dialer or that's not a solution?
Go to Settings --> Applications --> Manage Applications.
Select the 'All' tab (depends on the phone/android version) and scroll to the current dialer app.
Scroll down and press the 'Clear defaults' button. Now when you select dial, you should be prompted to select a particular app and also have the opportunity to set a new default.
Best Regards,
Paul