As DD said, suspension should only happen after a parent is notified, and it is verified that that notification was RECEIVED. A voicemail (and it doesn't sound like there's any way that was done in this case) or a letter is not sufficient. An actual CONVERSATION with a parent/guardian or communication that has a reasonably good chance it was with the parent (e-mail, using an e-mail address that is already on file and verified as a parents, granted a kid can hack into their parents e-mail and answer as the parent, but at least there's a reasonable chance it really is the parent) should be the only acceptable means of notification. Just telling a kid/teen that they're suspended without making sure the parents are notified is 1) opening up a whole shitload of liability and 2)guarenteeing that the kid has a free day to roam freely, regardless of how the parents would otherwise treat a suspension.
And this IS possible. When I was a teacher, each time I suspended a kid, I had an actual conversation with at least one parent/guardian in which I told the adult exactly why the kid was suspended. Some were easy to get ahold of, others not so easy, but I managed to track them down and talk to them (whether they wanted to listen was on them, but I got ahold of them if I had to call them at work or had to keep calling well into the evening. If I knew the parent wasn't able to answer the phone till late, I called late, but I always spoke with a parent).