This could be the world's simplest question, but I just got introduced to this system and I've spent a good deal of quality time with the 1200 page user manual already, trying to find the glitch.

I have DND turned off, but when other extensions try to buzz my extension, they hear a (sort of sirenesque, is there a word for that?) tone instead of getting through. What do I need to do to clear my line?

The phone rings on trunk calls, and I've hit DND-0 a bunch of times now to make sure (and the light's off!) and the MIC light is on, so I'm out of ideas.

Am I overlooking something obvious?