Most for what is offered would use the Branded user interface for customers to access. The rest would be very manually done on a separate website.

It seems HostGator.com Reseller Plan would get you WHMCS storefront that provides Integrated Hosting Storefront. Likely the majority of VOIPo Resellers already sell Web Hosting, and add VoIP to their offerings.

An API would go a long way into creating something on WHMCS that would be more seamless.

In the meantime, you will have to manually create the accounts. If the signup seems to work all the way to checkout, a delay to handle the product should work for now.