An acknowledge back (ack-back) pager is provide for use in a paging system which includes a central station which transmits a group of message signals to a group of ack-back pagers which are addressed as a group. The users of the group of addressed ack-back pagers indicate a response to their respective...http://www.google.de/patents/US4823123?utm_source=gb-gplus-sharePatent US4823123 - Knowledge back pager with frequency control apparatus
