Yes but there is a cost involved with this. No calls are free. It’s just they are going over your GWAN network, the GWAN network has a cost. With One Voice BT will route the call to a point where they have to forward the call to Off Net point and the charge will be based on BT standard pricing.
Example: You call my mobile number in Australia. Your call will come from the site over GWAN to Australia. When the call reached to Australia, the call is forwarded to local provider Optus for my mobile phone, this portion you need to pay. The cost is applied based on each country.