a solid state push - button dial circuit in which a push - button key pad uses a simple on - off switch for each key to mute the receiver and open the circuit to the transmitter while a dial tone is being generated . the circuit , when operating with the speaker phone , mutes the speaker phone when the key is depressed . the phone can be easily switched to a polarity - sensitive state in which the tone generator is turned on only if the proper polarity is present on the telephone line .