An authentication system is disclosed. The system comprises means for receiving an authentication request associated with the transaction wherein the request comprises data identifying a communication device associated with a user authorised to perform the transaction; means for sending a Mobile Application Part, MAP, protocol request message in response to the authentication request; means for receiving, in response to the MAP protocol request, data indicative of whether a communication sent to the communication device will be forwarded to a different communication device. The received data indicative of whether a communication sent to the communication device will be forwarded to a different communication device is used in determining whether to authenticate the transaction.