Here it's explained in wiki... All eris' on 1.5 do it most of you prob just don't realize it...
In
telephony,
sidetone is the effect of sound that is picked up by the telephone's mouthpiece and introduced (at low level) into the earpiece of the same handset, acting as
feedback. Sidetone in 19th century telephones varied until the
carbon transmitter was used which produced a loud sidetone that discouraged speaking loudly enough, and occasionally so loud as to throw the instrument into uncontrolled oscillation or "howling". Anti-sidetone circuitry incorporating the principle of the
hybrid coil brought sidetone under control in the early 20th century, leaving enough to assure the user that the phone is really working, and allowing the use of a unitized
telephone handset.
Without sidetone, users do not hear their own voice in the earpiece, and may think the phone is not working. Too much sidetone causes users to hear their own voice loudly. They may feel uncomfortable and lower the level of their voice.
Digital telephones lack the mechanical
acoustics and circuitry that created sidetone in older
landline phones, so digital phones include
electronic circuitry to reproduce the sidetone. Many
cell phones do not provide adequate sidetone.[
citation needed] Usability experts believe this causes some people to shout or speak too loudly when using a cell phone.
[1][2] Similarly, in military field operations in enemy territory, operatives can be encouraged to speak quietly by increasing the volume of the sidetone.
Sidetone is useful for people using handsets but can cause
audio feedback in
teleconferencing systems if not treated properly.