Forwards calls to Google Chat is an important option for people who are using Google Voice as free VOIP solution. In order to able to receive calling, forwards calls to Google Chat option must be activated so the VOIP adapter can acts as a Google Chat to receive calling, eg: OBi Devices. Without that, the phone will never ring when people call your Google Voice number.
If the Google Voice Account was newly set up, in default the "Forwards calls to Google Chat" option is inactivated. Google didn't provide any selectable option for adding the option to Google Voice.
Here is how you can add Google Chat to the "Forwards calls to" list.
Open Gmail using the same Google Voice account and call a phone number using the Google Hangouts feature on the lower left hand side.
If the Forwards calls to Google Chat Option under "Phones" does not show after that, then click on the chat option and change to the old "Chat" feature to convert it back.
Then dial a phone number and hang up.
Now the "Forward to Google Chat" option should be available in Google Voice.