I always keep my phone on Airplane Mode, but for some reason, I got charged the $10 when I made a call
to the French side. It was on WIFI also, like my other calls.

It took hours on the phone to get the charge off, but no one seems to know enough about it to tell you why.
Just check your bill, when you return.

It usually isn't a problem. when calling businesses on the island from our cell.
If there is no Whatsapp, I usually pay a small long distance charge.