Screen lights up and phone vibrates in 'Do not disturb’ mode

Hello all, I’m having a bit of trouble with the ‘do not disturb’ mode (usually present in the control centre). I could have sworn that, previously this mode used to prevent any incoming calls from ringing, vibrating or flashing on the screen when the phone was locked. However, this no longer seems to be the case for phone calls or other app notifications. Even when the phone is in my pocket, I can still feel it vibrating when ‘do not disturb’ is activated. I am an apple newbie, so I guess I’ve probably messed about with a setting and can’t remember what I’ve changed. Also, I recently upgraded my iOS from v14 to15 so I have the latest iOS version. Thanks and hope to get some help.