Originally Posted by mooney

No. You are under NO obligation to call them back and acknowledge it, or even to show up for the trip. They did NOT get ahold of you while you were on duty. They do that hoping to get someone gullible enough to call back. They probably left voicemails with 15 other guys about the same trip, and whoever calls gets stuck with it.
Now if it was on your schedule BEFORE you check out from your last trip that's a different story...

as a disclaimer, since this is a public anonymous forum, I would recommend calling a union rep today just to make it "official" and get it on record that it is an illegal assignment so when you show up for work next week you don't have a miss trip on your schedule. Guilty till proven innocent here...