Hello Disillusioned,
Unfortunately, I'm not sure whether the Back-up Calling feature is controlled by the mobile provider. I have previously considered carrying two devices -- one as a local data hotspot and the other using Wifi-calling. That feature would certainly be useful.
It's worth noting, though, that some Wifi-calling usage is considered roaming. 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).
While the feature doesn't technically seem to require Wifi to function, calls and messages would still require access to the networks via the internet gateway. I presume the same usage case for roaming with Wifi-calling would apply.
It's also of interest to note that the feature might likely not work with the Telus and/or Bell networks as they don't seem to allow Wifi-calling usage outside of Canada.
Hope this helps 😀
Cheers
