The school my oldest initially went to had this rule from the offset. I told them that my child would be having a phone on the commute because they do not reliably check the children in and out of school. From inside the school grounds, you just press a button to exit certain gates depending on the time of day. You don't use an individual card to swipe out.
The school stays open until 7pm. During this time, not only are there students and staff supervising and participating in various activities from organised sports (set time to leave) to informal drama rehearsals and library revision (can leave any time), the school premises is rented out to the community for other activities. The main one involved sports training for secondary school aged children, but one involved a small group of adults using a space for a group interest.
There was a register taken of the students attending extracurricular activities, but this was not digital so if you called reception after about 5pm, there may not be anyone there and they couldn't reliably tell you where your child was in the school, or even if they were there at all. They'd have to tannoy and ask them to come to reception if they were there.
Bear in mind, they may keep your child back for detention of varying lengths with no warning or they could give your child a detention, and then postpone it at short notice as the person supposed to be supervising the detention has more pressing matters.
I advise everyone to double check the schools are on top of this kind of thing before they agree to not allow their children a phone at all.