Hi ZuluAlpha,
I poked around a bit and couldn't find a "User" setting in which to mute the ringer. I even looked through the guides a bit to see if you could invoke the DND Feature Access Code, and nope, you cannot. (Though I would think being SIP, the device itself would be the one to say it is on DND) Surprisingly, I never had this question come up.
Thanks,
-Iron