Disclosed are various embodiments for detecting Signaling System 7 (SS7) redirection attacks by measuring call audio round trip times between phones. Such redirection attacks force calls to travel longer physical distances than usual, thereby causing longer end-to-end delay. Accordingly, various embodiments implement a distance bounding-inspired protocol that allows for securely characterizing the round trip time between two call endpoints. As a result, telephone users can reliably detect SS7 redirection attacks and protect the information contained in their calls.