There's more info about Trellis on the main site.
Error Correction - Trellis Encoding.
As regards to the Line Status Check in your router - thats not quite the same thing as what the ISP/BT can do.
Your router has some basic tests that it can do.
~ Check that sync is up
~ Check that ATM and PPP is up
~ Check connectivity - Some routers have a "home" ping test, which checks if it can ping say Netgear.com etc
~ and also something called BERT which is a
Bit
Error
Rate
Test. Which is the ratio of erroneous bits to transmitted bits.