Hello Honestguy,
Welcome to the community!
Sorry to hear you've incurred an unexpected roaming charge. However, the Fido Roam charges will only incur if your phone uses services -- make or receive a call, send a text message or use data abroad. You should note that 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.
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).
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.
The mobile providers only know usage. If a device is using services when its settings are set not to use them, that's an issue with the device. The mobile providers have no access to a device's settings to know what settings are enabled/disabled. I understand you may not have intentionally used services, however, if you had not enabled Airplane of Flight mode on your device, there is unfortunately no 100% guarantee your device won't use other services and trigger roaming charges.
For the sake of completeness, 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. 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).
Hope this helps 😀
Cheers
