The "No more prepaid certificates" is an informational message stating that you used correct activation credentials, but there are no free licenses in your license pool. For your information, each and every Zoiper Biz certificate for desktop comes with up to (3) three resets.
If you are moving Zoiper to a new computer, have changed your computer's hardware or reinstalled your operating system, you would need to reset your Zoiper Biz certificate.
To reset your certificate go to www.zoiper.com and click on "Customer Login" on the top right.
Here, you can login with the same username and password that you have received from our automated email message received, back when you have purchased Zoiper Biz.
If you do not remember your Zoiper credentials, you can use the password recovery option on the same page to reset the password as well.
Once logged in, you will see a list of licenses in use and will be able to reset these.