Your number can be kept for 7 days after its expiration or accidental deletion by Dingtone. You can get it back within those 7 days. (If you have owned the number for less than a month, we can only keep it for one or two days after you delete it.) After 7 days, the number may be purchased by another user and you cannot get it back.
Therefore, to own a phone number that can be used as your second phone number, it’s best to purchase it so that you can really own it.