They are probably using Google Talk to do that. There is an app for that but if I'm not mistaken it may use a different phone number than your current one. Not sure if the same can be done with Google Voice. You can check the website for more details. As far as making the Visual VoiceMail that the iPhone has natively on it to do that...I am not aware of a way to do so and did not see it in the owners manual available online or on the iPhone 4s....unless I overlooked it. You can also check the manual to see if any such feature is available.