Patent ID: 8755498

Claim:
A communication terminal with a direct dial-in function in a push button communication system, comprising: a polarity determining circuit connected to a pair of station lines; a primary acknowledgement signal transmitting circuit connected to said pair of station lines and to said polarity determining circuit; an extension designation signal detecting circuit connected to said pair of station lines and to said primary acknowledgement signal transmitting circuit; and a polarity establishing circuit connected to said extension designation signal detecting circuit, responsive to reception by said extension designation signal detecting circuit of a first digit at the start of an extension designation signal after detection by said polarity determining circuit that polarities of said pair of station lines are reversed, for establishing the polarities of said pair of station lines determined by said polarity determining circuit; wherein said extension designation signal detecting circuit includes a prescribed digit reception detecting circuit responsive to reception of the first digit at the start of said extension designation signal to output a detection signal; and said polarity establishing circuit includes a circuit responsive to said detection signal for establishing polarities of said pair of station lines determined by said polarity determining circuit.