If you do not already have a full Google Voice account, you must upgrade your account before receiving phone calls in Gmail. This process is free and gives you access to additional Google Voice features such as one number that rings all your phones and voicemail transcription.
- In Gmail, click the Call phone link in the Chat list.
- From the Call window, click your current account balance and select History or Add credit.
- Once in Google Voice, click the Upgrade your account button.
- Complete the upgrade steps including picking your new Google number.