To be able to receive phone calls within GMail, you need to have a Google Voice account. Such a thing is offered free by Google, but so far, not all countries are covered. So many of you might need to wait a while before being able to benefit from Google Voice.
Google Voice has the characteristic of being able to ring multiple phones on an incoming call. You can configure your account to receive the phone calls while you are on your computer, within your GMail account, or to receive them on any phone of your choice.
Note that when you make GMail calls without a Google Voice account, the number that shows on your correspondent’s phone is Google’s default number for that: 760-705-8888. When you call with a Google Voice account, your correspondent will see the Google Voice number, which can be one given by Google or one you have ported yourself. Here’s a video on how to set up Google Voice, and here’s my review of it.