We have a couple of VG350 gateways registered via SCCP to CUCM and from time to time some analog phones have no dial-tone when they go off hook. Random 4-5 per day from a total of 400. They start to work again after 2-3 minutes so the problem doesn't last. We have no idea what to check. The only thing we noticed is that all the ports have some unrecognized packets received. I don't know what this means and if this can be the reason for the missing dial-tone.
Thanks for the reply but fortunately I found someone on the forum with the exact same problem. A simple enough but very unlikely problem in a normal Cisco IPT environment. It is very specific to these VGs. See it here: http://shar.es/1mstbu.