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").
[1][2]
 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