Hello FidoUserJK,
Welcome to the community!
I understand receiving that message can be disconcerting if you had not intended on using Fido services abroad. Merely receiving the Welcome messages does not necessarily mean you've incurred roaming charges. I was able to confirm that to be true here. 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). The mobile providers only know usage.
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 receiving calls using the local cellular networks.
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).
Fido Roam is already enabled on eligible plans. There is no activation of Fido Roam other than using services. If people wish not to use Fido Roam, they would need to opt-out of the feature. If you opt-out of the service, any usage would be on a pay-per-use basis. Those rates can be found 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.
As mentioned, receiving the Welcome messages does not necessarily mean you've incurred roaming charges. However, if other services were used when your device connected to the foreign network, that usage would likely have triggered roaming charges. 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.
Hope this helps 😀
Cheers
