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Hi,

If you set the DND on the phone, usually the phone do not send anything to the PBX and the PBX doesn't actually know, it's the phone that replies with a "BUSY" message if the PBX tries to call it.

You can set up the phone so that instead of setting the DND on itself, when you press the DND it sends a signal to the PBX and the DND it's setup on the PBX, to do so you need to enable this parameter

http://wiki.snom.com/Settings/dfks 

And then you would need to get into the Identity->Features menu on the web interface and set the DND on and off codes.

If you do so, when you press the DND the phones sends those codes to the PBX and it's then the PBX job to set the phone on DND.

The codes depends on the PBX you are using and not on the phone, the PBX needs to be able to understand that the phone is asking to be put in DND.