Can there be an option to enable backup calling? The only thing needed is just a button in carrier settings, to enable backup calling for SIM, just like what we do for VoLTE and VoNR. Android used to have a backup calling option, but now it is controlled by Carrier Config. It is extremely for some Carrier like T-Mobile, they support Wi-Fi Calling but default carrier config does not include Backup Calling enabled (default backup calling settings seems to be disabled for all carriers, but it is actually available for almost all carriers)
It can also be modified with Pixel IMS / Pixel VoLTE Patch, and seems not to be complex. But these method all requires Shizuku, which means user have to root or always enable ADB, both of these will break the security system of GrapheneOS, and also, Google have much more limit on Pixel IMS resolution, so maybe we can add a native option for it?