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Telephone system with multiple extension telephones

  • US 5,090,052 A
  • Filed: 04/09/1991
  • Issued: 02/18/1992
  • Est. Priority Date: 05/18/1989
  • Status: Expired due to Term
First Claim
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1. A method for communicating between a plurality of telephones over an extension line connected to an outside telephone line, each of the telephones having a selectable-frequency modulator and demodulator, each of the telephones having an on-hook state and an off-hook state, the method comprising:

  • detecting a signal on said extension line;

    determining whether internal communication is present;

    selecting a carrier frequency for transmission receiving an indication of a called extension unit;

    sending, from a first telephone, a first signal indicating an extension call, indicating a first carrier frequency for transmission by said first telephone, and indicating the called extension unit;

    receiving, at a second telephone, said first signal;

    transmitting, from said second telephone, following said step of sending, a second signal indicating an on-hook condition of said second telephone;

    transmitting, from said second telephone, a third signal, after entering an off-hook state, indicating said off-hook state and indicating a second carrier frequency for transmission by said second telephone;

    selecting, at said second telephone, said first frequency for demodulation and said second frequency for modulation; and

    selecting, at said first telephone, said second frequency for demodulation and said first frequency for modulation.

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