Hello Donito-Mojito,
Welcome to the community!
Firstly, what plan-type do you have? In addition, what phone do you have? You should note some plans (ie Basic Plans) do not allow for roaming.
It would be important to note that the US has shut-down their '3G' networks (see here). If your phone and plan are not voice-over-LTE (voLTE) capable, you might not be able to use voice or messaging services. As well, since the US has shut-down those networks, they no longer offer US roaming services for pre-paid plans.
If you have a plan other than the Basic plans or pre-paid, it would already have Fido Roam included. The daily charge for roaming usage in the US is $12 per day and Internationally is $15 per day (see link below). 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 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).
Hope this helps 😀
Cheers
