Do not disturb function: question about while iphone is locked?

I am missing very important calls as the iphone is not always ringing. It rings sometimes but not others. I have read all articles and the moon icon is off. I am concerned it has something to do with the settings. In settings the option to silence gives me two: always and while iphone is locked (it says incoming calls will be silenced while iphone is locked) It doesn't give me the option to uncheck. It's permanently checked.

Is there a problem with the phone?