Communication between a telephone and a computer system

A method and apparatus provide communication in a telephone system connected to an external phone line, wherein the telephone system has a telephone line, a telephone, a phone interface unit coupled to the telephone and the telephone line, a computer system, and a computer interface unit coupled to the computer system and the telephone line. A bias voltage on the telephone line is detected to determine if the telephone is on-hook or off-hook, a ringing voltage on the telephone line is detected to determine if an external call is incoming, and a first carrier transmitted by the phone interface unit is detected. A second carrier is transmitted using the computer interface unit under control of the computer system. Detection of the bias voltage, ringing voltage, and/or first carrier is communicated to the computer system. An off-hook condition is indicated if the line bias voltage is less than an off-hook bias threshold voltage. An incoming ring is indicated if the ringing voltage has an amplitude greater than a first voltage and a frequency in the range of a ring frequency. The first carrier is indicated as being present if a signal is detected at a first carrier center frequency and having an amplitude greater than a first carrier threshold voltage. The second carrier is indicated as being present if a signal is detected at a second carrier center frequency and having an amplitude greater than a second carrier threshold voltage.