Is it possible to buy a number without saving a credit card on file?

In this article:

To purchase a new number, you will need to have an active account with a credit card on file. You will be charged for your new number when it becomes active in your account. Your monthly bill will be automatically charged to your card on file.

What happens when my new number activates?

Once your number ports away, all billing will stop.

To remove your card from your account after your number ports away, you can close your account:

How to cancel or close an account