Abstract: In a regional telephone network system offering centrex services, a closed user group is distributed as subgroups to several different telephone exchanges. According to the invention, the management of user groups can be implemented in such a way that a table-type subscriber database is stored in each exchange with subscribers of at least one closed user group. The database is associated with this user group and contains all information necessary for implementing centrex services so that when the call control of the exchange detects on the basis of the subscriber data that the subscriber belongs to the closed user group, it utilizes the subscriber database when establishing a call.