Hello Aliceanton,
Welcome to the community!
Sorry to hear you incurred a long-distance charge for that call. However, the issue you've mentioned isn't the fault of the mobile providers. It happens across multiple providers. WhatsApp seems to be the culprit. While it's not mentioned on their website, it appears as though the app will automatically switch to using mobile airtime minutes (within the app) if it deems the internet connection is too slow for voice calls (see here and here). I do understand that you might have made the same call for that period of time without incident. Unfortunately, WhatsApp appeared to deem the internet connection unsuitable for that particular call and switched to mobile minutes. It should also be mentioned that other messaging apps may also behave in a similar manner.
One suggestion would be to enable Airplane or Flight mode and manually enabling Wifi prior to making the call(s). Doing so should prevent the app from switching to mobile services if the Wifi internet connection is not sufficient. However, it also means that you wouldn't be able to make WhatsApp calls using a mobile data connection.
Phone companies cannot divert calls from within apps to cellular calls. They have no control over how apps work. The mobile providers do not decide which calls go over their networks or how they transmit. That determination is made by the phone. With an adequate internet connection, WhatsApp calls are usually sent via the internet. On the other hand, if the internet connection is not sufficient, WhatsApp apparently switches to mobile airtime minutes. The mobile providers merely connect calls which the phone request to be made. If WhatsApp makes a call using data, the providers will transmit data; if WhatsApp makes the call using cellular networks, the providers will transmit it as a call over the cellular networks. Unfortunately, in your case, it appears as though WhatsApp made the call over the cellular networks. Not connecting to the networks (ie Airplane or Flight mode) should prevent that from happening in the future.
You should note these forums are community-driven and not intended as a venue for customer services. If you would like to further discuss those charges, you would need to re-contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
Cheers
