A telephone number used for account registration, two-factor authentication, or as contact information. Phone numbers are strong identifiers that can link accounts across platforms and be looked up through caller ID services and contact import features. ### Related weaknesses - [[SOWEL-9. Saving Contacts]] - [[SOWEL-6. Reusing Personal Data]] - [[SOWEL-22. Providing Contact Information]] ### Related techniques - [[SOTL-9.1. Import Contact]] - [[SOTL-9.2. Check Caller Identification Apps]] - [[SOTL-9.3. Check "Who Called" Sites]] - [[SOTL-22.2. Check Whois History]]