Hello Honest_Alberto,
Welcome to the community!
You are correct in your understanding that if you have a plan other than the Basic plans or pre-paid, it would already have Fido Roam enabled. 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. In addition, the daily roaming charge is inclusive of data and airtime so there are no additional charges for those on top of the roaming fee.
It should be mentioned that while receiving SMS would not incur the roaming charge, any 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. It should further be noted that iMessages and RCS messages are sent (and received) as data and not as text messages (SMS).
If you did not wish to use Fido Roam, you would need to opt-out of the feature. In doing so, any usage would be on a pay-per-use basis. You can view those rates here. If you wished to opt-out of Fido Roam, you would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
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. Alternatively, people have also removed their Fido SIMs card entirely, However, enabling Airplane / Flight mode or removing the SIM card would also prevent your device from making and/or receiving calls or sending and/or receiving SMS using the local cellular networks.
As well, 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
