A user authentication system that analyzes call forwarding information obtained from telecommunication networks, such as through the use of Signaling System No. 7 (“SS7”) protocols, to detect the possibility of fraud. In response to a request to access a network-accessible service, the system performs an initial authentication of provided user account credentials. The system then obtains a telecommunication subscriber identifier that is associated with the user account. Prior to performing additional device-based user authentication, the system obtains call forwarding information for the user. The obtained call forwarding information is then evaluated for potentially fraudulent call forwarding configurations. For example, call forwarding to certain call forwarding numbers, or the use of different call forwarding types, may be indicative of fraud intended to undermine further user authentication.