What I have found works, and works well. Your Polycom phones should be at Bootrom 4.2 and Firmware 3.1.3RevC (no later!). Have MOH disabled in Polycom phones. Have MOH enabled on the SBC. Apply this patch for sipXbridge.
From the sipx console,
chmod +x runme.sh
You should check that
are brand new files, backup copies will be in the saved-files directory relative to your patch in the event you need to get back to where you started..
Now restart sipxbridge (ITSP TRUNKING) and sipXconfig from services and see how it works!
This post will be updated to show MOH on in the Polycom phones, the necessary steps and bandwdth.com configuration, and will all move to the sipfoundry wiki.