Charged international phone usage for Whatsapp calls

I have consistently used Whatsapp for years and love it. However, something very odd has happened in the last two months and I got a
 call from my provider (Verizon) saying there have been an unusually high 
frequency of international calls made by my phone and I had racked up $1200 
in charges. I figured it was fraud but then checked my usage online and all calls
 (to two regular numbers I call in Europe) show up as regular 
international calls. This began at the beginning of September (2014) and I
 can't understand what is happening. The rate for these calls is about $1.29/min.

I live in the US and have many contacts in Europe, so like many other people, I have great use for Whatsapp. I was
 especially excited back in the spring of this year when they announced free
 calling, and I used it a lot. It doesn't make sense to me why I was 
successfully making calls for free for the first few months, and then all 
of a sudden in September, they've been going out as if I was calling
 internationally through my carrier. There was no mistake on my part as I haven't changed any of my methods since the beginning of using Whatsapp calling.

Has anybody else had this experience? Is there some setting that got switched in a new iOS update that makes their calls just run normally? (I know, highly unlikely but I don't know what else could have happened).

Thank you very much for any help!