Telephone system and bell sound detecting method thereof

A telephone system comprising:

• a corded telephone set having the CODEC function for converting a digital signal inputted from a digital line to an analog signal;
• a cordless telephone set having a talk switch;
• a relay lifter;
• and
• a bell sound detector for detecting a non-electrical vibration of a bell sound in said corded telephone set, wherein said relay lifter comprises:
• a hook switch ON/OFF unit for providing ON/OFF control over a hook switch of said corded telephone set according to a first control signal responding to said talk switch of said cordless telephone set;
• a voice switching unit for switching connection of an aural signal between a corded telephone set and said cordless telephone set according to a second control signal responding to said talk switch of said cordless telephone set, wherein the operating of said talk switch of said cordless telephone set turns ON or OFF said hook switch of said corded telephone set, and said voice switching unit switches connection of an aural signal from said corded telephone set to said cordless telephone set;
• and
• a detection signal transfer unit coupled to said bell sound detector for transferring a detection signal from said bell sound detector to said cordless telephone set when an incoming call is received, wherein said cordless telephone set compares a voltage waveform pattern of a detection signal over time to a predetermined waveform pattern and outputs a bell sound from said cordless telephone set when said comparison differentiates a bell sound pattern.