How do I recover a deleted phone number on iPhone?

It depends where you deleted it from. If you deleted it from your Recent Call history, contact your carrier as they may have a saved history of it. If you deleted it in the form of a Contact, you can go to icloud.com/recovery, sign in with your Apple Account credentials, then choose a previous archive of your contacts to restore that includes the deleted number. Otherwise if you deleted it elsewhere, please specify.