The schools I am involved with have added a whole new appendix to the safeguarding policy to cover these extraordinary circumstances we are currently in.
There is nothing normal/routine at the moment, and unfortunately this means that parents may need to adapt how they deal with school. I have to agree with @Hercwasonaroll if a parent was reacting in the way you have in some of your posts @mayawaya I would be concerned if a teacher didn't raise a safeguarding concern. Doesn't mean social services/the police would be banging on your front door, but I would want a note made. And yes it could mean that the school could communicate with you by email, but maybe they could arrange a phone call with the child, depending on age. I assumed the OP was simply asking why the schools were phoning, as obviously they wouldn't have been doing this when schools were properly open, not that it was causing then undue anxiety. OP I apologise if I am wrong.
I think it was in the news the other day that only 5% of vulnerable children are currently in school (and that is 5% of known vulnerable children). Teachers who care about their pupils worry about this. Schools are constantly working on how they can keep communication open with these families and also ensure they are not missing any other children who currently haven't had to be on their radar.
In normal times all schools have a procedure when a child doesn't turn up for registration. I think normally if there has been no contact from a parent to explain why the child isn't coming in, contact will be made from the school within 15 minutes of registration. If the school cannot contact anyone on the contact list quite often someone will go to that child's house to ensure everything is ok, and in most cases it is, but not always.
Now, the majority of children are not in school. Schools don't have this regular daily procedure to check that a child is ok, but these children who are at risk are still at risk, probably more so, and other children are becoming at risk.
Parents please don't begrudge the phone calls, it may help a child/family somewhere. And if you struggle with the phone calls, speak to the school.