As a service that provides numbers to use temporarily, we do offer numbers on prepaid plans that can expire.
When your number initially expires you have 48 hours to restore without having to pay a fee. After 48 hours you are to able pay a $2 fee to restore your number in-app with a new plan. After 7 days without restoring your number, it becomes no longer available to be restored to your account.
PS - You can set reminders to make sure your numbers never expire again! Follow this link to learn more: How do I set number expiry notifications?