When I was working for a CLEC as a contractor a couple yrs ago, I ran into this issue once with a client who had an older rotary wallmounted phone. I used my Ideal Industries Smart6 banjo/tester to confirm the line polarity was reversed. So I pulled the phone off the wall and corrected the polarity in the jack. Fixed the issue. Previous techs hadn't been able to fix this for the client. Other jacks had been wired correctly. It was an apartment building.
I found most of the time the LEC didn't bother to install the CLEC's lines with correct polarity, whether it was in the LEC's side of the NID or at the CO.
