Hello Kcaspo,
Welcome to the community!
Firstly, Fido Roam is automatically enabled on Data, Talk & Text and Talk & Text plans. If you did not wish to use the feature, you would need to opt-out of Fido Roam. In doing so, your usage would be on a pay-per-use basis. You can view those rates here.
In order to prevent unintended roaming charges, it is generally recommended to disable data and roaming data. However, you should note that disabling those settings 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 also generally recommended to enable Airplane and/or Flight modes in order to prevent unintended roaming usage. You could then manually enable Wifi when required.
Since your query is regarding Wifi-calling abroad, it should be noted that while using Wifi for regular internet access will not incur Fido Roam charges, there seems to be some misunderstanding regarding Wifi-calling. Some Wifi-calling usage is considered roaming. 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).
**edit** The Fido Roam charges will incur if your phone uses services -- make or receive a call, send a text message or use data abroad.
While receiving SMS would not incur the roaming charge, it's worth mentioning that the data contained within MMS (incoming or outgoing) would be considered roaming data and charged accordingly. You should note that MMS are not solely restricted to pictures or video messages. Messages with subject headers or group messages etc are also considered MMS. Disabling MMS should prevent that data transfer. Though, it should further be noted that iMessages and RCS messages are sent (and received) as data and not as text messages (SMS). Even if the text message received is SMS, if your device is configured to provide Delivery and/or Read receipts, those would be sent as SMS or data and considered roaming usage.
Unanswered incoming calls and having them go to voicemail should also not incur roaming charges (see here and here). However, it should be noted that checking your voicemail would be considered roaming as one must call the voicemail service. In addition, it should also be mentioned that the Voicemail-to-text feature notifications would be sent as MMS (roaming data). Similarly, I believe iPhone's Visual Voicemail would use data (roaming, if abroad). **
Hope this helps 😀
Cheers
