I've had this same issue for a couple of months now. My wife, has the Asus zenfone 9 128gb/8gb ram, no issues. My 256gb/16gb ram zenfone 9, call issues galore. Many people calling me, it simply sends them to voicemail. For some it just doesn't connect, and of course, I don't see anything on my side. Tried all software resets, sim change, no luck. Opened a ticket with Asus Taiwan as it could be hardware issue, but their warranty coverage and customer support is so spotty, that it's taking forever.
This started happening to mine as well around 2 weeks ago, but I can't be sure since I don't really get that much calls, I just noticed it 2 weeks ago. I can't make/receive calls but texts and mobile data works fine. I also have full signal. Tried using my sim on another working phone and calls work fine, just not on my Zenfone 9.