Make calls from Gmail :

You can make a call from your Gmail account from your Google Chat list or your email.

Make a call using your Google Chat list
You'll know you're using Google Chat if the green, yellow, or red circle is next to a person's name on the left. If you're using Hangouts in Gmail, you'll see a green or gray circle on the bottom right of the person's photo. Learn more on how to call a phone using Hangouts.

If your voice calls in Google Chat don't work after an upgrade to Chrome 42, you need to use Hangouts to avoid this problem.

1-Log into your Gmail account.

2-In your Google Chat list, click the Call phone icon . 
3-Use your mouse or keyboard’s number pad to dial the number you want to call.
4-Click Call.

If you have a Google Voice number, it will appear on the recipient's caller ID. If you haven't set up a Google Voice number, a generic number will appear to the person you're calling.

Make a call from an email
You can make calls directly from an email you send or receive.

1-When you open an email with a phone number in the text, the number will be blue.
2-Click the phone number in the email.
3-In the dial pad that appears, click Call. 

Via : https://support.google.com/chat/answer/3379791

& https://www.youtube.com/watch?v=_-DzpAg0SdU