My wife’s iPhone 12 Pro Max also has this same issue since upgrading from an iPhone 7 in Nov. I also upgraded from android to just the iPhone12 at the same time and dont seem to have the issue. Along with missing Visual VM (hitting *86 they are waiting at Verizon VM system) she is also at times not getting the inbound call and does not get any indication of a missed call.
We keep applying the latest iOS hoping the fix the issue. Verizon also sent us a single booster to install in the house because we only had 2 bars but problem seemed like it was fixed but still continues. I have about 4 technical support tickets opened with 3 of those escalated to higher level of support. Next I am going to push to see about getting a replacement phone.