Secondary. Too many vulnerable students to risk not messaging every single day.
Phone calls are a start, but anybody can pretend to be a parent - an older 'boyfriend', a sibling or 'friend' - so all calls have to be backed up by an email and/or a letter. It's not unheard of for kids to get into Mum's emails or set up a fake parental account, either, so the slightest bit of doubt and verification is sought by the other contact methods.
No absence is left undocumented. Doesn't matter if it happened last September, there will still be verbal requests from tutors, emails, texts, calls from attendance staff and involvement from Heads of Year and Assistant Head if no acceptable response is received.
Might feel like it's all a massive hassle, but nobody wants to be the person who didn't persist and a teenager disappears/turns up in hospital. Or, as happened to a lad in my class at school many years ago, dead for 8 hours by the time his parents came home and found him.
In addition, LACs and other children known to SS/At Risk will often have external agencies calling every day to check attendance and what efforts have been made to contact parents/carers if they are absent.
Knowing exactly where everybody is and being able to show it is a fundamental Safeguarding requirement. Learned from awful, awful experiences - the original reason for the phrase 'lessons will be learned'. We learned. And that's why your phones are pinging by 9.30am.