Hi @mjw913,
I hope you've a Webex Developer Sandbox Account from here - https://developer.webex.com/docs/developer-sandbox-guide and if not already, you can request one.
Once you've the Sandbox account with the Free Test Org credentials of Control Hub, you can add that User to your licensed Org first and then add the license for that Sandbox Developer Test account as well.
But yes, you need to first purchase the Cloud Calling licenses for your Paid Org since it's not available for free and more details regarding that can be found here - https://www.cisco.com/c/en/us/products/unified-communications/cloud-calling/index.html#~market-leadership
Hope it helps !
Regards,
Sandip