However, the problem is I'm not in the U.S., then every call to GV is an international call at high rate to me :/
If you need a callback only when calling from one of a short list of numbers, you can set the OBi AA to call you back without answering at all, so you wouldn't get charged.
What country are you in? There may be a way to get a free or inexpensive DID that could be used to trigger the callback.
Do you have web access from where you are making the call? If so, a click-to-call service may be your best bet. Otherwise, a callback service such as Fonworld might work.
Please post the details of your application (calling from where, to where, using mobile/smartphone/landline/payphone/etc.), approximate anticipated monthly minutes.