Google has been working on Smart Forwarding for dual-SIM phones for a few years now. Back in 2019, XDA found reference to the feature, which would automatically forward phone calls coming into one of your SIMs to the other, if the original SIM didn't have service. Now it seems like Google will finally be moving forward with this on Android 12 for Pixel phones.
Smart Forwarding is now present within the settings app in the new Android 12 beta on the Google Pixel 5, though not currently visible; I was able to access the screen with Activity Launcher. Unfortunately, I was not able to actually enable the feature, but did grab a peek at what the UI looks like. All it currently shows is the two numbers for the SIMs and the option to enable the feature. This is likely a super early implementation of it, as this is hidden as part of the first beta.
There's no word on if this is just being worked on for a future Pixel device or if it will also come to existing generations of handsets (like this Pixel 5) when officially released and available. If you want to know a little more about what's coming in Android 12 or what is in current betas, check out our hands-on with Android 12.