I get a lot of calls from people claiming their Polycom 330 and 550 phone’s DND function doesn’t work.

The Polycom doesn’t update the Trixbox with the fact that they’ve put the phone on DND. So here's my fix:

On your Trixbox, edit the /etc/tftpboot/sip.cfg file.

Add this section:

<softkey softkey.3.label="DND" 
softkey.3.action="^*76" 
softkey.3.enable="1" 
softkey.3.precede="0" 
softkey.3.use.idle="1 />

Enable feature 18 (extended functions):
feature.18.enabled="1"