This happens more than it should really, in my previous job I helped with training new starters certain buts and bobs, in a group of 20 at least 3/4 would have to be spoken to about time keeping.So do I, but I can cope with the concept that a 9am start means 9am not 1120am and putting your phone on silent and in your pocket isn't that big a deal.
Considering they are new, you would think they would at least try and make an effort to be on time.