Mutual authentication techniques are described in this patent document. For example, when a first person calls a second person, neither of them know that the other person is who he or she says he or she is. Thus, after a second person receives the call, the second person is asked to authenticate himself or herself using a user device. After the second person logs into his or her account, the second person can input on the user device a one-time passcode to authenticate the first person. The user device sends the passcode to an authentication server that allows the first person to send back the inputted one-time passcode to the second person. Upon receiving the inputted one-time passcode, the second person can use his or her user device to indicate that the one-time passcode is correct so that the second person can be authenticated to access the first person's account.