Is there a way to retrieve a deleted phone number on my iPhone?

If the phone number was stored in iCloud contacts:

… Restore contacts stored in iCloud on iCloud.com - Apple Support

Otherwise, from a local iPhone backup either to iCloud or a Mac or Windows PC. (Restoring that will overwrite all subsequent changes, so either back up again before restoring the backup and revert to the newer backup later, or revert to the backup. Be very careful here with your backups too, as a new backup can overwrite a previous one.)

Otherwise, no.