I will say I have had plenty of similar issues across my iPhone devices. For a variety of reasons these phones fail to make calls. Drop calls or can’t dial 911. I suspect voice calling itself has become network deprioritized or still has trouble selecting between calling tech.

I worked on dialer for pixel and and am deeply familiar with these problems. Voice call tech is easily one of those mostly worthlessly convoluted spaces in mobile. That being said while I was there we put a huge priority on emergency calling — however the exec overseeing the space often complained about how hard dialer was and bemoaned all the required work because it never helped her promotions.