iPhone 11 Pro Max keeps freezing and restarting every few minutes

My iPhone keeps freezing and restarting about every few minutes. I have closed all the apps in the background, have not installed any new app, deleted a few apps (the latest ones - i keep new apps on the main screens for a while before moving them to a folder) but still the phone keeps freezing, then restarting with an Apple logo. It is on the latest firmware and all apps are up to date. I have scheduled an appt with Apple Store and i am assuming they will reset it but i cannot backup the phone as it keeps freezing. I am missing extremely important calls as it freezes even in the middle of phone calls. Can someone please suggest a way to at least keep the phone running for the phone calls?

Also, how to i backup my phone if it keeps freezing?

Any response will be appreciated. Thanks!