The telephone division of the Netherlands P.T.T. has until recently used a revolving panel operation to register outgoing and incoming telephone calls. Write in-questionnaires were being used. They provided useful but far from perfect information. The data produced covered only about 70% of all calls made and the estimates of their duration were poor. These problems are now being solved by supplying panel members with NIPO's question/answer machines attached to the telephone, rather than with the questionnaire block notes previously used. These machines record essential occurrences inside the telephone and the monitor screens project short batteries of questions immediately after each telephone conversation. Panel members reply by pushing the relevant answer-buttons. The data so registered, both the technical records and the answers to questions, are transmitted to NIPO's computer over the telephone wires during the night.