This invention allows customers and other users of the Public Switched Telephone Network (PSTN) to enable, disable or modify special services provided by the network. The PSTN is defined here as all equipment world wide (land-based, radio or other) enabling ordinary telephone users to connect to other...http://www.google.de/patents/US5917817?utm_source=gb-gplus-sharePatent US5917817 - User invocation of services in public switched telephone network via parallel data networks
