Hello DrFMS,
I understand you intend on using an alternate eSIM while in the US. I also understand you do not wish to incur roaming charges on your Fido line.
I also understand there might be an option to disable roaming on your primary SIM. Unfortunately, most modern devices do not have an option to disable roaming entirely (except for Airplane or Flight mode). You should note the disabling roaming data does not prevent roaming for calls and messages. I further understand you are not able to enable Airplane or Flight mode on the device since you wish to use another SIM. Unfortuntely, there are no standards on how mobile manufacturers implement the dual-SIM functionality in their devices. If there are no options to completely disable your Fido SIM card, it could be possible to incur Fido Roam charges during your trip.
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. 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. It should also be noted that RCS messages are sent (and received) as data and not as text messages (SMS). Even if a received text message is SMS, if your device is configured to provide Delivery and/or Read receipts, those would be sent as SMS or data and considered roaming usage.
The mobile providers only know usage. They have no access to, nor control of, a device's settings. If devices' SIM connects to a foreign cellular tower and uses services abroad -- even when unintended -- that usage would be considered roaming.
Fido Roam is enabled by default on most current post-paid plans. If you did not wish to avail of the feature, you would have to opt-out of Fido Roam. In doing so, any usage would be on a pay-per-use basis. Those rates can be found here. Alternatively, if you did not wish to use any services, you would could request to have roaming blocked on your account. You would need to verify which option is applied to your account as some customers had assumed they requested a total roaming block only to find pay-per-use usage abroad (opt-out Fido Roam).
You should note the forums are community-driven and not intended as a venue for customer services. If you wished to discuss your matter and/or opt-out of Fido Roam (or apply roaming block), 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.
Hope this helps 😀
Cheers
