Can I get back an Onoff number I deleted?

Yes, it's possible to get back a number you deleted by subscribing to a new plan, subject to the quarantine rules.

During the quarantine period: After deletion, the number is temporarily reserved for you. It appears at the very top of the list of new numbers available for purchase in the app, and you're the only one who can see and repurchase it.

After the quarantine period: The number is released back onto the open market. If it no longer appears in your purchase list, it has already been assigned to another user and can no longer be recovered.