The invention relates to a mobile system and especially to detecting, in
the mobile system in question, the use of terminal equipment having a
copied mobile equipment identity. In the invention, a database containing
records is created, each record containing an international mobile
equipment identity associated with a mobile station and an international
mobile subscriber identity, a check is made to see whether the database
contains a record which contains a mobile equipment identity
corresponding to the mobile equipment identity transmitted by the mobile
station, but whose mobile subscriber identity does not correspond to that
transmitted by the mobile station, and if yes, at least a signal is
produced, indicating that the mobile equipment identity is possibly a
copied one.