If I'm not mistaken an ACARS message is a valid form of notification now. All they have to do is send you a message with "Crew Acknowledge Required."
I'm pretty sure Delta has it in their contract that they can only be officially notified of an extension in person. I've watched one guy escape down the jetway stairs and into the bowels of the airport before someone could show up to tell him.
I looked and didn't really find anything in the contract that references that. Anyone have a quote from the contract on this?