Q: how do i retrieve a phone number accidently erased from my iphone6

The following link has instructions on how to restore using an iTunes or iCloud backup. Note that a restore will replace all of the data on your phone with the data from the time of the backup. There isn't a way to restore just Contacts from a backup. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If you had the Contacts synced to iCloud (i.e., if Settings > iCloud > Contacts was turned on), you can use a computer and log in to https://www.icloud.com/ and then go to the Settings app there and in the Advanced section at lower left there is an option to Restore Contacts. Using that will allow you to select an archive date and will then restore your Contacts on all of your devices that have Contact sync turned on. It will also make a new archive of the contacts before it does the restore.