If you have an automatic recurring payment
Login to your account, create a basic plan subscription for your loved one (if you haven't already), and then upgrade their subscription to the personal number plan. You will be able to choose the area code of their personal number.
If you are making one-time payment with credits
If you paid using credits, your loved one will need to run a command on their end to upgrade their account to use a personal phone number, click here for instructions.
*Upgrading to the personal number plan done by your loved one running a command will assign them a random area code, you only get to choose the area code if you upgrade an automatic recurring payment.
Important: Upgrading to a personal number plan is only possible after getting the basic plan.