GV doesn't handle "early media", which refers to voice sent or received before a call is "answered". In the case of a changed/disconnected number, early media is used so the caller is not charged for the call. On the toll-free number, it's set up (as part of agreement between customer and carrier), so the callee is not charged for the IVR portion of the call.
Your OBi can connect to multiple providers and you can choose a different one for calls that don't work correctly with GV. I recommend Callcentric, primarily to have reliable 911 service, now that your landline is disconnected. 911 is $1.50/mo.; you'll also pay $0.0198/min. for any domestic calls you make.
If you are sure that you don't need 911, you might use Localphone; most US calls are $0.005/min. and calls to toll-free numbers are not charged at all. Quality and reliability are decent, but not as good as Callcentric. There are other free choices for toll-free calls; some don't even require you to sign up.