Abstract: In accord therewith, at least one identification number (PIID) for identifying a private network (PISN) for access to a first private branch exchange (PBX1) to a subscriber (BS) who was registered and a mobile subscriber telephone number (MSISDN) that was sent into the public mobile radio telephone network (PLMN), is stored in the public mobile radio telephone network (PLMN). A routing number (PRIN) is offered in the mobile radio telephone network on the basis of the incoming mobile subscriber telephone number (MSISDN) and is checked with reference to the stored identification number (PIID) as to whether the call destination lies in the area of a private branch exchange (PBX2) of the same private network (PISN) to which the first private branch exchange (PBX1) belongs.