I've also had problems with my iPhone 12 Pro and my wife's iPhone 12 and 14.2. Issues include random dropping Verizon LTE signal (toggling airplane mode usually restores signal, at least briefly) and no WiFi calling. I spent hours on the phone with senior Apple support and Verizon level 2. I've tried everything others have suggested. Apple won't confirm there's a problem. Verizon says they have an "Alert" and their techs are working with Apple on this. I originally bought my phones unlocked from Apple, and they've replaced the phones. I finally returned them and purchased new phones from Verizon. Same issues on all six phones!
My 12s worked for the first two weeks we had them using the old sim cards from our 11s. Verizon says older sim cards will work on their LTE network, but not on the 5g network. I can confirm this. The first phones shipped with iOS 14.01. Everything went to hell after updating to 14.1 and continued with 14.2 and even into beta 2 of 14.3.
It's puzzling that not everyone with 12s or even everyone who updated to 14.2 - even those with older phones - has these problems.
At this point, I don't expect a solution until Apple resolves the problems in a future iOS update. Giddy-up Apple!
sillydg
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Update to previous post about cell signal weakness (Verizon, iPhone 12, iOS 14.2.1) and WiFi calling not working:
After further conversation with Apple, I believe that what Apple is calling "the 1-bar symptom" is caused by "the choice of network thresholds by the Carrier which can force the device to stay on an LTE band with a weaker signal." Apple sent a field engineer to my house to collect logs using multiple devices including several iPhones and an Android device. Apple then contacted Verizon to optimize their configuration for my location. I have a stronger signal since they did this. Obviously the carriers (not just Verizon) need to do this for their whole network, not just one location.
Regarding my WiFi calling issue, Apple suggested that I disable QoS on my router, and now I can use WiFi calling. On one of my iPhones the change was instant; on our other iPhone, we had to remove the SIM card and then replace it to get WiFi calling working. Further, Apple says the the "no-bar symptom" was also caused by having QoS enabled.