If it can be shown they are slamming based on just entering a number into the online checker then that is very serious.
This is unlikely, since why bother to do this, they really only stand to gain if you do something provide bank account details. Suspect most slamming arises from the mall stands, and door to door sales.
I am somewhat sceptical of people, as have come across a few cases where people claim to have been slammed, but reality is they placed order, and tried to cancel the day before activation or similar. i.e. not all cases are as simple as people make out.