An authentication method for a telecommunications system comprising a computer network and a telephone network that includes receiving, at an Authentication System across the computer network, data indicative of an authentication request. The request is typically made by a provider in connection with a user (e.g. by a provider to obtain authentication from a user as the result of an authentication trigger detected by a provider system). A telephone call is establishing by the telephone network between the Authentication System and a telephone device with a telephone number associated with the user. A PIN is received from the user at the Authentication System during the telephone call. It is then determined if the received PIN is valid. The authentication request is authenticated if the PIN is determined to be valid.