The present invention relates to a method and system for locating a mobile terminal addressed in a location service request by an overlay identity. A database (60) is accessed to obtain a first routing information of the mobile terminal based on the overlay identity, and the first routing information is then used to access a sub-scriber database (40) in order to derive a second routing information and a cellular identity of the mobile terminal (10), based on which a location service of the cellular network is initiated. Thereby, a LCS clients of the overlay system (e.g. IMS) can be supported in current LCS services to enhance their functionality.