Data access in a telephone system

Method of improving the data security of a data service connected to a telephone network in a telephone system including subscribers, subscriber equipment and telephone exchanges, wherein the data service is connected to the telephone exchange by a data access, the call number of which is chosen by the subscriber when starting the data call, and in which system it is possible to form closed user groups, the inside calls of which are different as regards the way in which they are set up from calls made outside the user group and from calls received from outside the user group, whereby data concerning the user group is stored in the subscriber data of the subscribers belonging to the group,

• characterized in that
• a closed user group is formed which includes the data access and the users of the data service which it connects with the telephone system, and data indicating membership in the user group is added to the access data of the data access,
• when starting a data call, the subscriber sends a request for set-up of a data call connection as a call of the closed user group, and if the subscriber has the right to calls within the user group:
• the data call is routed to that telephone exchange which has a connected data access whose call number was chosen by the subscriber,
• in the telephone exchange, the user group data of the call is compared with the access data of the data access,
• such incoming calls are barred which come from outside the closed user group of the data service, and
• such incoming calls to the data access are set up which are within the closed user group of the data service,
• authentication of calls from the telephone system through the data access point, are accepted, and the subscriber who has taken contact is given access to the data service.