Welcome to the community!
Are you referring to using Wifi-calling? If so, you should note that Wifi-calling is different from other types of Wifi-based calling apps such as WhatsApp or Skype, etc. With app-based calls, the entirety of the calls are transmitted via the internet. In addition, those types of calls require both parties to use the same app. If the receiving party does not use the app, there are charges for connecting those calls. The reason there are charges for calls outside of the app is that those calls would terminate via the telecommunications networks (cellular or landline).
With Wifi-calling, the calls do not transmit entirely via the internet. Wifi-calling calls are routed through the cellular networks (see graphic here). Rather than connecting to a cellular tower, your phone is using Wifi to access the cellular networks. The services still use cellular networks. Calls are treated the same as if a call was made over the cellular network because they are being made over the cellular networks. Wifi is just a different means to connect to the networks.
Making calls to non-Canadian phone numbers with Wifi-calling will be the same as if you made the same calls using your cellular plan. If you do not have any International calling or add-on, you will be charged long-distance for those calls.
On the other hand, if you had attempted to use an app, you should note that some apps might automatically switch to using mobile airtime minutes (within the app) if it deems the internet connection is too slow for voice calls (see WhatsApp example here and here). It should also be mentioned that other messaging apps may also behave in a similar manner.
Phone companies cannot divert calls from within apps to cellular calls. The mobile providers do not decide which calls go over their networks. That determination is made by the phone. They merely connect calls which the phone request to be made. In your case, it appears as though WhatsApp is making the calls over the cellular networks. Not connecting to the networks (ie Airplane or Flight mode) should prevent that from happening.
It an app was the culprit, 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 app calls using a mobile data connection.
Hope this helps 😀