Hello Roameo,
Welcome to the community!
The Fido Roam charge incurs when your device uses Fido Services abroad. That is, the charge will incur when you make/receive calls, send text messages or your device uses data outside of Canada (see here). If you require making or receiving calls using your Fido number, that would be considered roaming usage.
You should note that disabling data and roaming data does not prevent roaming for voice and messaging. In addition, some manufacturers might allow some data usage to occur despite those settings (see here). That's why it is generally recommended to enable Airplane and/or Flight modes to prevent unintended roaming usage. You could then manually enable Wifi when required. However, enabling Airplane or Flight mode would also prevent your device from making and receiving calls using the local cellular networks.
I understand you intend on using Wifi when available. If you have a supported device and plan, Wifi-calling may be an option. That said, you shoud be aware of its roaming usage. There seems to be some misunderstanding regarding Wifi-calling . I'm not sure how people have gotten the impression that Wifi-calling does not use the networks. However, that is not true. While the calls and messages do not transmit via cellular towers, they still use the networks via the internet gateway (see image here). The cellular towers and Wifi are merely alternate means of accessing the networks. Calls and messages would not be able to complete or get sent/received without the networks. As such, using Wifi-calling is technically using Fido services. However, they have allowed certain usage to be free from additional charges. All the messages and calls you receive (from anywhere in the world) will be taken from your plan's voice minutes and messaging limits. As well, all Wifi-calling messages and calls you make to a Canadian phone number while abroad won't incur long-distance or roaming charges (see link above).
If you wished to opt-out of Fido Roam, you would need to contact customer service. If you opt-out of the service, any usage would be on a pay-per-use basis. Those rates can be found here. 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
