A landline telephone apparatus comprising: a handset; a DTMF signal generation unit configured to generate a DTMF signal based on a telephone number; a line interface unit configured to transmit the DTMF signal to a line; a line volume measurement unit configured to measure a volume of the DTMF signal input from the line; a control unit configured to control a sidetone volume, a transmission volume, and a reception volume, wherein the sidetone volume defines a volume of a sidetone signal, the transmission volume defines a volume of a transmission signal, and the reception volume defines a volume of a reception signal; and a storage unit configured to store a maximum value of an acoustic echo generated at the handset and set values of the sidetone volume, the transmission volume, and the reception volume, wherein the control unit is configured to perform a first processing of: estimating a line echo amount based on a volume of the DTMF signal generated by the DTMF signal generation unit and a volume of the DTMF signal input from the line; calculating a howling noise threshold value based on the maximum value of the acoustic echo and the set values of the sidetone volume, the transmission volume, and the reception volume; and calculating a difference between the line echo amount and the howling noise threshold value as an adjustment requisite amount and lowering the set value of the sidetone volume by the adjustment requisite amount when the line echo amount is larger than the howling noise threshold value. a handset; a DTMF signal generation unit configured to generate a DTMF signal based on a telephone number; a line interface unit configured to transmit the DTMF signal to a line; a line volume measurement unit configured to measure a volume of the DTMF signal input from the line; a control unit configured to control a sidetone volume, a transmission volume, and a reception volume, wherein the sidetone volume defines a volume of a sidetone signal, the transmission volume defines a volume of a transmission signal, and the reception volume defines a volume of a reception signal; and a storage unit configured to store a maximum value of an acoustic echo generated at the handset and set values of the sidetone volume, the transmission volume, and the reception volume, a maximum value of an acoustic echo generated at the handset and set values of the sidetone volume, the transmission volume, and the reception volume, estimating a line echo amount based on a volume of the DTMF signal generated by the DTMF signal generation unit and a volume of the DTMF signal input from the line; a volume of the DTMF signal generated by the DTMF signal generation unit and a volume of the DTMF signal input from the line; calculating a howling noise threshold value based on the maximum value of the acoustic echo and the set values of the sidetone volume, the transmission volume, and the reception volume; and the maximum value of the acoustic echo and the set values of the sidetone volume, the transmission volume, and the reception volume; and calculating a difference between the line echo amount and the howling noise threshold value as an adjustment requisite amount and lowering the set value of the sidetone volume by the adjustment requisite amount when the line echo amount is larger than the howling noise threshold value.