There is provided a method of authenticating a public land mobile network (PLMN) to a mobile station (MS). The PLMN provides a circuit switched access network to the MS, and the MS and a trusted service, TS, have established a security context. The initiation of a voice call or initiation of sending or receiving of a pushed message by the MS (S10.1) is detected, and the TS generates a response authentication token using a session identifier and the security context (S10.7). The TS sends a response comprising the response authentication token to the MS such that the response is delivered to the MS over a signalling channel of the PLMN using a mobile subscriber integrated services digital network-number (MSISDN) associated with the subscriber or subscription to direct the message (S10.9). The MS authenticates the response using the response authentication token, the security context, and the session identifier (S10.12) in order to authenticate the PLMN.