There is stored in the public mobile network (PLMN) at least one identification number (PIID) for identifying a private network (PISN) for access to a first private branch exchange (PBX1) which registered a call for a subscriber (BS) and send a mobile subscriber number (MSISDN) in the public mobile network. A routing number (PIRN) is prepared on the basis of the incoming mobile subscriber number (MSISDN) in the mobile radio network, and checked in relation to the stored identification number (PIID) to see whether the call destination is in the area of a private branch exchange (PBX2) of the same private network (PISN) to which the first private branch exchange (PBX1) belongs. In the latter case, routing information (RIN), which establishes the call destination after release of the call in the public mobile network, is transmitted to the first private branch exchange (PBX1) which subsequently establishes the connection to the call destination using the received routing information (RIN) in the private network (PISN).