Also just started having this issue recently (in the past 2 weeks). Theoretically could have been longer and I just didn't know that I missed a call/text. Then one day my spouse was calling/texting me repeatedly and it went straight to voicemail. The calls did not show up as missed calls, and the texts did not show up until 24hrs later when they all delivered at once.
Have a Pixel 8 and AT&T. Have never had wi-fi calling turned on. This is NOT an AT&T issue, as I have another device (iPhone XR) on AT&T and I can still receive calls and texts when my Graphene phone will not. Note that I can still make calls and successfully send texts during times when I am not receiving calls or texts.
It is an intermittent failure. Restarting seems to help, but that could be a coincidence.