On Sun, 25 Aug 2013 11:18:34 +0100, Andy Cap
wrote:

If the line still sounds distorted, insist on an engineer's
visit. If you'd done this, they'd be hard pressed to prove it was on
your equipment and therefore apply any charge.

I'm afraid these days BT often apply the test of "no fault found on
our equipment" to justify a callout charge. This would be especially
so if when the fitter tested during the day no fault was apparent as
seems to be the case here.