Those who are not having pre-existing full Google Voice account, they will have to upgrade their account to receive Gmail phone calls. Upgrade procedure provide you extra Google Voice features and is available for free. You will get a single number which will rings all the phones and voicemail record.
Process to upgrade-
1. Click on link of Call phone link under Chat list in Gmail.
2. Under Call window you can click on present account balance and choose History or Add credit.
3. Now you need to press Upgrade your account button one time in Google Voice.
4. Now you can follow upgrade steps and can pick a new Google number.
You are required to have valid US number to upgrade and use full Google Voice Account.
To receive phone calls in Gmail:
1. You are required to make minimum one outbound call through your Gmail.
2. Under gear icon go to Settings in Google Voice.
3. Beside Google Chat select the check box in Phones tab.
You can click Edit button for editing settings for forwarding phone call. You can disable Gmail as a forwarding phone simply by unchecking the box next to Google Chat.
While receiving call on Gmail you will get a call window with a ringtone to alert you. At times sound of ringtone is not audible which can be due to muted device sound so ensure sound is on. You only need to click Answer button for accepting the call. If you wish not to receive the call you can ignore it by pressing Ignore button.