FIRWARE,
The only time I have seen something like this was on a TA5000 chassis in a Remote Terminal cabinet. On the TA5000's POTS and ADSL+POTS modules, after a period of time, some of the POTS ports across various cards installed in the chassis would go into an off-hook state, but no call was active.
The issue was the TA5000 chassis ground wire was not "attached" to the ground bar of the cabinet. The AC/DC rectifier, suppling the -48VDC, ground bar "was attached" to the cabinet ground bar.
Once the TA5000's chassis ground was connected to the cabinet ground bar the issue never occurred again.
So, I would confirm the NetVanta's AC ground is stable / valid compared to the 19" equipment rack which should also be grounded. If either ground is bad, that would be similar to the TA5000 situation explained above.
Hope this helps.
Regards,
BMULLAR