Hello Mkster84,
Welcome to the community!
Firstly, you should note that modern phones do not have a setting to disable roaming entirely. They do have settings to disable roaming data. However, disabling roaming data does not prevent roaming for voice and messaging.
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). While receiving SMS does not incur roaming charges, receiving MMS would likely incur charges since they are sent as data. 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.
I understand you have disabled your cellular data, however, you should also note that some phones might consider certain system data differently from user data and continue to allow the former usage despite data and roaming data settings disabled. For example, while disabling data and roaming data should prevent data usage on foreign networks, some mobile manufacturers do seem to allow some data usage despite those settings disabled (see here). I am not sure whether iPhones allow MMS to transmit with those settings disabled.
Even if the text message you received was SMS, if your device is configured to provide Delivery and/or Read receipts, those would be sent as SMS and considered roaming usage.
In order to prevent unintended roaming charges, it would be generally recommended to keep Airplane or Flight modes enabled for the duration of trips. You could manually enable Wifi when required.
Generally using Wifi for regular internet access will not incur Fido Roam charges. There seems to be some misunderstanding regarding Wifi-calling, though. 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
