The hangup apparatus uses an inadvertent off-hook detector circuit designed to detect the off-hook "howler" alert signal transmitted to a telephone by the telephone company when the telephone has been off-hook for a period of time without any user activity. The detector circuit provides an automatic...http://www.google.ca/patents/US4922529?utm_source=gb-gplus-sharePatent US4922529 - Automatic hangup apparatus for restoring service to off-hook telephones
