Just google sidetone!
And this is a totally different thing than what the user was asking about.
Sidetone is using so that you hear your voice as your speak so you have the ability (by way of feedback from your own voice) to control the volume of your own voice.
I do remember so cellphone phones that did not provide even that (along with not providing Full Duplexing with the caller at the other end), mostly done to make the phone cheaper. I always hated that, because you never knew how to control your voice.
Here is the explanation from Wikipedia:
Telephony
In
telephony,
sidetone is the effect of sound that is picked up by the telephone's mouthpiece and in real-time introduced at a low level into the earpiece of the same handset, acting as controlled
feedback. Sidetone in 19th century telephones varied until the
carbon transmitter was used, which produced a distinct sidetone that discouraged speaking loudly enough, and occasionally so loud as to throw the instrument into uncontrolled oscillation or "howling". Sidetone is inactive when phones of any kind are running in speakerphone mode, due to perpetual and almost immediate feedback looping. 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. In cellular technologies, one of the many benefits of sidetone-enabled phones is that a user knows a call has been dropped or ended if he or she no longer hears sidetone.
Usability studies done at RIM (manufacturers of BlackBerry smartphones),
LG and
Motorola have demonstrated that a lack of sidetone has a tendency to make the user of a phone or cellular handset characterize it as dead or disconnected. In the same battery of tests, it was found that the presence of sidetone prevents users from needing to examine the device's display to determine if a call is still active. First introduced in the
StarTAC handset, almost all cellular handsets manufactured by Motorola have sidetone, though its level of feedback ajustable by the user. Too much sidetone causes users to hear their own voice loudly which is why it is not standard on all cellular handsets and leaves the decision to incorporate sidetone up to the manufacturers. In usability studies prior to the launch of Apple's first-generation
iPhone, users were quoted as feeling uncomfortable when the amount of sidetone is too high and will lower the level of their voice unnecessarily. Currently, the iPhone does not have +6% - +9% sidetone,[
clarification needed] while almost all Android, Palm, and Windows mobile devices do.
Digital telephones lack the mechanical
acoustics and circuitry that used physical wiring to produce sidetone in older
landline phones, so digital phones include
electronic circuitry,
software and
firmware to reproduce sidetone. Many
cell phones do not provide adequate sidetone despite general agreement among leading industrial design & usability experts who claim it is an important feature in cell phones, perhaps even more so than for land-lines because of the less predictable acoustics one will encounter while using a cellular phone.
Almost all land-line (wired and wireless) phones have employed sidetone, so naturally it was an expected convention for cellular telephony but is not standard by any means. Usability experts believe that lack of adequate sidetone causes some people to shout or speak too loudly when using a cell phone (this behavior is often referred to as "cell yell").
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Sidetone is valuable for the hearing impaired. The amount of sidetone typically found on land-lines is 8%, and is 4% for cellular phones. Sidetone can be, and often is, amplified for land-line phones for the hearing impaired. In
VOIP technologies such as
Skype, sidetone has been experimented with but has not been formally adopted by software or hardware & accessories creators. Several software packages and wiring workarounds have been developed that replicate sidetone, but feedback looping remains a problem.
Rob