Same problem here.
iPhone 12 Pro Max bought as a gift for my birthday, it drops out during calls (as in the call still connected, yet the person you’re speaking with drops out for 10-20 seconds) and I am unable to get service greater than 3g.
I have called Apple Support and also had the phone tested at a Genius Bar appointment, I was told it could be my service provider. Since, I met with service provider, 5g is active on my account, they also replaced the SIM to be completely sure and they have no issues on their end.
I am currently Hot Spotting from my daughters phone (she has my previous iPhone 11 Pro Max) and I have no signal issues whatsoever. Internet, socials, email etc work perfectly.
APPLE, please fix this immediately and admit the issue. The Apple stores must be inundated with Genius Bar appointments about this.
On a side note, I also asked for a screen performance test to be done and it reported no issues. However, continually, if I need to close a window via tapping on the top left corner of the screen, it is unresponsive. Often requiring me to force close the App or window I am using.
See photo of me currently Hot Spotting to use my phone. 