Telephone apparatus

A telephone set, where the state of an own-telephone-set is on-hook, the off-hook state of a parallel-telephone-set is detected by comparing telephone-line voltage value with a first standard voltage value. In the state where the own-telephone-set is off-hook, the off-hook state of the parallel-telephone-set is detected by the following method, i.e., the telephone-line voltage value is measured in detail and a second standard voltage value for judging the off-hook state of the parallel-telephone-set is determined and the voltage value is stored in place of the first standard voltage value while the own-telephone-set is off-hooked, then if the line voltage value exceeds the second standard voltage value, the parallel-telephone-set is regarded as off-hook. The telephone set having the above circuit structure can control not to send an acknowledgment signal when an alert signal is input into the telephone line in the state where the parallel-telephone-set is detected to be off-hook accordingly, the data sounds of the acknowledgment signal and caller data are prevented from being noise heard through the parallel-telephone-set.