The present invention provides a method involving a femtocell in communication with a secure core network such as an Internet Protocol Multimedia Subsystem (IMS) network. The method includes receiving, from the femtocell and at a first secure entity in the IMS network, a global challenge including information indicating a random number. The method also includes receiving an authentication response computed by a mobile unit based on the random number and the first key known by the mobile unit and not known by the femtocell. The method further includes determining, at the first secure entity, that the random number is a legitimate random number provided to the femtocell by the IMS network.