The Google Chat "pseudo forwarding phone" was only used by Google Voice to send forwarded calls to the Google Chat system (and unintentionally, to third parties logged in as Chat clients, such as your OBi box).
Google Chat is obsolete, and it is being shut down on the 15th. The Chat entry on your forwarding phones list is now completely useless, and so it may be disabled and deleted. It has no interaction at all with any actual telephone numbers you list as forwarding phones.
Google Chat's successor product, Hangouts, works automatically as a forwarding destination, and it does not require an entry on your forwarding phones list.