I did some reading on this with the OPPO phone it has a couple different features on it for DND.
To my knowledge and understanding from what I have read online a phone will ping the towers when it is searching for information.
This will vary on each phone and depending what notifications that you have set up.
Flight mode and sleep mode switch off the receiving end so that you are not disturbed or the signal cannot come through the phone, just as you are on an aircraft. I believe OPPO has a feature that in sleep mode you can add contacts or apps to override this but nothing else, so if you wanted your parents to be able to contact you in event of an emergency they still could but no further notification would come through.
When a phone is first switched on it will start searching for the carrier, IE Telstra and when it locates the signal there is your ping.
Google Maps, Facebook Gmail etc anything that you have notification alerts set to on and or location services again will be searching for that signal to locate you or to send you a notification / email / sms etc.
I think of it in a way of your telco / wifii/bluetooth signal sending information to your phone based on your settings.
If you are in DND, flight mode, sleep mode I believe this cancels this ability to ping the phone. As soon as the phone is switched on and taken out of these status it opens the lines of reciving communication therefore will ping.
I am not tech nut however from my reading this is what I understand only and all IMO.