a key telephone system includes a plurality of key telephone sets having speakers and tone signal generators and communicating with each other through extension links . the key telephone system further includes a first memory , a second memory , a main cpu , a sub cpu , an mf signal generator , and a speech path switch . the first memory stores availability of the extension links . the main cpu discriminates whether all extension links are available according to the storage contents of the first memory in an extension call mode . the mf signal generator signals to a calling key telephone set that all the extension links are busy when the main cpu discriminates that all the extension links are busy . a second memory stores information on each calling and called key telephone set if the extension call is in a state where the mf signal generator signals that all the extension links are busy . the main and sub cpus cooperate to perform call control of the called key telephone set using the tone signal generator and the speaker in the called key telephone set according to information stored in the first and second memories .