A method and apparatus are provided for authenticating a user attempting to establish a service which uses SIP. The user registers with the SIP server by providing the digital otoacoustic signature of the user. Thereafter, when the user attempts to initiate a session through the SIP server, the SIP server sends an Authorization Request message to the SIP client of the user. The SIP client reads the user's digital otoacoustic signature, generates a response based on the digital otoacoustic signature, and embeds the response in a second Invite message sent back to the SIP server. Meanwhile, the SIP server determines an expected response, based on the digital otoacoustic signature registered by the user. If the response provided by the SIP client matches the response expected by the SIP server, the SIP server allows establishment of the server. The invention is of particular use with IP telephony services, since end user devices used with such services are ergonomically well suited for reading of otoacoustic signatures. The invention may also be used to authenticate an intended recipient of an SIP call.