VoIP Confusion

It seems I still don't have this whole phone thing sorted yet. If someone can help me make sense of the following incident, I'd really appreciate it.

Cellphone Support Rep: Hello, you've reached <provider> support. How can I help you?
Me: Hi. There are some charges on my most recent bill I don't understand. The international roaming charges accrued on <day> -- amounting to nearly $100 -- are for calls I didn't make.
Rep: Oh, I'm sorry.
Me: I called previously about this and just got a follow-up email from support. They say the numbers are owned by Bandwidth.com and represent VoIP calls. I've never heard of Bandwidth.com and don't have an account with them. I use Skype from my computer when I want to make VoIP calls. I don't even have a VoIP app on my phone.
Rep: I see. I'll remove the charges immediately. We're very sorry.
Me: Thank you.
Rep: Is there anything else I can help you with?
Me: That's it. Thanks. <hangs up>

As soon as I hung up, I realized that I do have a VoIP application on my phone: Google Voice. After a little digging, I learned that Google Voice uses Bandwidth.com. And from my Google Voice history, I saw that <gasp> I did in fact make those calls. Whoops.

The calls, as they appeared on my cellphone statement, had different numbers than the ones I dialed, and they appeared to be to random locations like Florida and DC. So my confusion is understandable. Besides, isn't the whole point of VoIP to avoid being charged extortionate international voice roaming rates?

So, what went wrong here? I'm certain I had a wifi connection when I made these calls. And even if the calls had switched to the cell network, I have a data plan and this is IP traffic. It's not called Voice Over IP for nothing. This should be covered by my data plan, not my voice plan, right?

Right?