I'm assuming it'd be free.
Why? Many VoIP providers charge for international still. Grant it they are very low rates but at some point a cross-over has to happen still.
Does AT&T, Comcast or other VoIP providers give free unlimited international calling with phone service?
That said, this probably won't require an OTA, but maybe.
Your account has a feature on it called WiFi Calling and if you drill into it the thing specifically says USA/America, my bet is they just upgrade the feature code which allows the phone to actually pass the international call.
Domestic they give free because it technically rides VoIP anyway and you using your WiFi actually helps keep half the load off the network as only the receiving party puts load on it now.
International involves other companies/parties etc. who charge for this long distance type calls.