I just contacted Obihai support & they said you can't do what I want to do.
In order to dial **2 you need a dial tone first, so I would have to use the Phone 2 port on the Obi202 to make an outbound call if someone is already using the Phone 1 port when I pick up the phone.
I should have mentioned that the Obi202 is plugged into a walljack & all the other phones in the house work off that.