Oh man, did I go through this hell with a lot of MVNOs like two years ago. Pushed me to buy T-Mobile's most expensive plan. Something I actually regret to this day now, because I'm not sure if I got any real tangible benefit from it compared to all the money that went down the drain. However, now that I think about it, I don't feel like I've been getting too many ghost calls as I used to. It's hard for me to measure because in the last two years, I've been getting less overall calls as more of my colleagues transition to just texting.

I doubt QCI levels are affected by this. I think Call handshakes are QCI 5 and the calls themselves are QCI 1 but im not an expert.

Have you tried disabling 5G and dropping back down to 4G? I heard that the transition from VoLTE to VoNR might still be sketchy and not fully solidified. Just turn off data altogether and see for a while to see if that improves things.

Maybe porting your primary number to a cloud PBX service like Google Voice or some business oriented service? When someone calls your number, the carrier-to-carrier SIP handshake happens on Google's redundant enterprise servers instead of targeting your sleeping iPhone. Google's servers instantly answer the routing request, hopefully preventing a timeout. GV then forwards the call to your iPhone's GV app using a standard data push notification, hopefully waking the phone reliably?

Just throwing ideas out. If you've tried these already, apologies for the redundancy.