Raven,
You must have one forwarding phone listed in your google voice account. So, if you only have one phone listed, and you terminate your home phone service, you can't delete that number from the account. You can either change it (new number verification needed by google voice), or you can just uncheck it as a forwarding number and leave it in the account unused.
as RonR stated, add the google chat as a forwarding phone and use it by itself to ring the Obi.