https://wccftech.com/future-android-version-might-allow-forward-calls-between-sims-using-smart-forwarding/

The AOSP (Android Open Source Project) has never failed to give us an insight into what’s going on behind the scenes at Android.  It also allows anyone to modify the code to their liking and often gives us sneak peeks into some upcoming Android features. Though not all of them make it to a final release, some of the ideas presented there are interesting. For example, an earlier commit showed us how Android R would allow users to store their documents such as drivers licenses digitally. XDA Developers came across another interesting commit today. It mentions a feature that can be loosely described as ‘smart call forwarding.’

The feature is intended for dual-SIM phones, which have only just marked their presence in the North American market. If one of your numbers happens to be unreachable, Smart Call Forwarding will kick in and will redirect all your calls from an unreachable number to the working one on your device. It requires two SIMs to be present in your device, and one has to be unreachable. Once those conditions are met, users will be able to trigger the functionality from the Android UI. Here is the feature description from the commit that sheds more light on it.