Sidetone is enabled by default on Dreamwave Phones running release 126.96.36.199 (D28P) & 188.8.131.52 (D26P) onwards.
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. The phone network must ensure that not too much of the caller's voice is fed back into his or her receiver. This is achieved by phasing the signal so that some cancellation occurs in the network before the signal is fed to the receiver.
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. Callers who hear no sidetone may consider the phone "dead." Very little sidetone will convince callers that they're not being heard and cause them to shout. "Can you hear me now?!" Too much sidetone causes callers to lower their voices and not be heard well at the other end of the line. This also makes for a very unpleasant call.
A telephone on a short loop with no loop compensation will appear to have too much sidetone, and callers will lower their voices. In this case, the percentage of sidetone is the same, but as the overall decibel level is higher the sidetone level will also be higher.
With the advent of digital phones, and standalone voip phones, this concept has changed dramatically. Each manufacturer has their own theories on the best levels of sidetone. Some manufacturers will also eliminate sidetone entirely.
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. Usability experts believe this causes some people to shout or speak too loudly when using a cell phone. 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 if not treated correctly mainly in teleconferencing and when taking calls on speaker.