Communication terminal adapted to direct dial-in service and method of establishing polarity

A communication terminal with a direct dial-in function in a push button communication system, comprising:

• polarity determining means connected to a pair of station lines, for determining polarities of said pair of station lines;
• primary acknowledgement signal transmitting means connected to said pair of station lines and to said polarity determining means, responsive to reception of a call signal after said polarity determining means detected reversal of polarities of said pair of station lines, for transmitting a primary acknowledgement signal to a line;
• extension designation signal detecting means for detecting a first digit at the start of an extension designation signal from an exchange, after transmission of said primary acknowledgement signal by said primary acknowledgement signal transmitting means;
• and
• polarity establishing means, responsive to detection by said extension designation signal detecting means of the first digit at the start of said extension designation signal, for establishing polarities of said station lines determined by said polarity determining means wherein;
• said extension designation signal detecting means includes means for detecting reception of a signal indicating the first digit at the start of said extension designation signal;
• and
• said polarity establishing means includes means, responsive to detection by said detecting means of reception of the signal indicating the first digit, for establishing polarities of said station lines determined by said polarity determining means.

In a communication terminal connected to an exchange through a pair of station lines and having a direct dial-in connection function in a push-button communication system, a polarity establishing method for establishing polarities of the station lines, comprising the steps of:

• detecting reversal of polarities of said pair of station lines;
• after said detecting step, transmitting a primary acknowledgement signal through said pair of station lines;
• after said transmitting step, receiving a signal designating the first digit at the start of an extension number through said pair of station lines;
• after completion of said receiving step, establishing polarities of said pair of station lines;
• and
• after completion of said establishing step, receiving a signal designating a remaining digit of said extension number;
• wherein
• said step of receiving a signal designating the first digit at the start of an extension number includes the step of receiving, after said transmitting step, a signal indicating the first digit at the start of said extension number, through said pair of station lines.