The current catch-all system is counterproductive.
I think for the attendance letters to be taken seriously and stop harassing people who are not (supposedly) the target, like some PPs have said, the way things are recorded and what kind of absence counts towards the total needs to change.
Authorised absence for medical appointments where an official letter/confirmation has been provided should absolutely not be counted in the 'bad' absence stats. It should be counted in the same way school trips are - recorded, but not affecting the overall attendance percentage. By recording it as having happened, the data should only be used to help understand why a DC may be falling behind, but not as a stick to beat them with. I often wonder how schools get away with the way DCs are discriminated against for medical appointments in the attendance figures, as surely this is highly likely to correlate with disabilities?
The daily phoning in is unacceptable where the absence is planned, documented and the duration broadly known, eg for an operation. This would be considered harassment in the workplace, and is not acceptable in schools.
After that, it really isn't good enough to have letters going out automatically when attendance hits a certain level. There needs to be some human involvement and common sense applied. Perhaps below a certain percentage, the school's attendance officer and LA's officer must have a chat first to decide if there is a good/understandable reason why the DC has been absent so often. Then possibly, like many workplaces would do, a supportive referral to occupational health should be considered. Surely the goal is to improve attendance and the default starting point that nobody wants their DC to miss lots of school (I appreciate some don't care, but it should be 'innocent until proven guilty'). Letters threatening legal action should be at the bottom of the list and an active choice by a human once genuine absence has been decided to be unlikely.
And now that I've mentioned common sense: 9yo DD1 got a late mark last week because her school bus broke down. We live in an outlying village some distance from the school (so few other options to get there) and the bus company insists on sending coaches that are on their last legs - yes, she was indeed late, but I'm not sure it is fair to record it in a way that it is a mark against her name when she had no control over it, school were very aware of what had happened (there were nearly 50 DCs on board) and that it was a genuinely unavoidable delay. There, again, needs to be either the option to not count lateness in such circumstances, or it be marked in a different category of 'lateness' that doesn't contribute to the poor attendance grade.
I also do what a PP described, booking appointments around registration times (when I get the choice - not often). It's ridiculous, as no more or less school time is missed as a result, but it 'plays the game' and keeps her attendance percentage high.
DD1 was really upset earlier this year when she had an occupational therapy appointment with a non-negotiable time. The purpose of the appointment in the main was to find reasonable adjustments to reduce DD's pain and discomfort at school, so she could be a more active learner. She didn't want to go because it would mean losing her 100% attendance status - this shouldn't be playing on her mind!