Speaking for myself, I had a persisten problem with a loud hum on the line (system grounded, updated, tested at office/no hum there).

In the end Tech support sent a modified line card (This was a known problem). Problem went away.

We also installed a 40 where line 1 would drop the call after a few seconds (no variations in line voltage). We called and they sent us a replacement system. Again, problem went away.

I'd be inclined to follow tech support...