The current backup calling feature will only activate if the data SIM is not in roaming status. However, in practice, when traveling, people may purchase international data SIMs, which are always in roaming status.
In this situation, Apple devices could still use "Wi-Fi Calling over Cellular Data" to make calls, but Samsung devices cannot, so I have to prepare a separate phone to enable a hotspot.
And actually, the description of "Data switching and backup calling" didn't mention anything about the roaming limitations, which could be considered misleading.
