Humans can get through fine. And so far, as far as anyone knows, they can get in and out. Supernaturals can go in, but they can't get out. The mechanism is not entirely known at this time, but if someone attempts to call out and warn anyone of what is happening, they are unable to get the words out. If their intention is to do that from the getgo, they can't even start dialing/typing.

Edit: Today's a big day for me, so I am not going to be writing the first post until the evening.