I can't tell you how many SPG properties I've been in this year with NO DND signs in the room! I call, but if it's late and I want to sleep I put a post-it (always travel with them) over the lock writing a note about "service later" or none at all. Once in a while a hotel will replace that note with a fresh DND sign.

I wish all properties would do what some of the LC have: switches to turn on/off a light at the door saying DND.