Abstract: A mobile telephone network stores a list of short dialling codes in association with each user's identity so as to define the short codes which each individual user is permitted to use for providing access to a data conversion device which gives access to a private network. When a short code is dialled on a mobile station, calling data comprising the short code together with caller identification data derived from the mobile station are transmitted over the radio link to the mobile switching center. Prior to routing the call, the identity of the dialling station is authenticated. The short code in the calling data is then compared with the permitted list of short codes to determine whether the user is permitted to make the call. If the caller is permitted to make the call, the call is routed to a destination corresponding to the data conversion device. Thus, only permitted users may access the data conversion device.