vigorphone 350 Firmware Version: 12202.26.1.05C
If I adjust levels under phone settings-voice settings, when i click o.k. it pops up a warning, click o.k. phone does not restart and settings not changed tried in firefox and ie.

Any ideas? or any other way of decreasing the handset audio?

If you are using your 350 phones with a 2820 IPPBX 3.5.5 there is a good chance that these settings are now auto-provisioned by the router.

I had the same issue with the codecs that now have to be set up by SSH/telnet on the router to change your phone settings.