How can I retrieve a phone number that I deleted from my iPhone?

If you have deleted a phone number from Recent Calls, it unfortunately cannot be restored. If you have deleted a Contact and your contacts are synced with iCloud, you may be able to restore it from the iCloud web page.

• Visit iCloud.com

See here how to restore your contacts in iCloud:

If you accidentally deleted your calendars, bookmarks, or contacts from iCloud - Apple Support

If you delete anything and do not have a backup before you did that, it is gone. That is what "delete" means.