A few months ago I had an intermittent fault (serious noises while making a call), always found to be non existent by their automated checking system which is next to useless, it took three visits from engineers before they finally swapped the line from the pole to the exchange. Fortunately it was happening at least twice when I was talking to them so they couldn't very well deny it and there was no charge. What would have happened if they hadn't heard it themselves I can only guess.
Greg