I only call BT if I cannot resolve my issue online or it is a complaint. My experience is 100% apalling when I call. Only the expectation of even more problems in the change prevents me changing provider.
Customer feedback only allows happy or unhappy options - there is no option for my blood is boiling, I am apoplectic or I need to lay down - which is always my experience.
Idea - everytime your customers rank your telephone customer service as AAAAAAAAAGH!! let an independent assessor award financial compensation up to the level of refunding all of their costs for that year for which your marginal cost is close to zero - i.e. most of them - and with perhaps at least a flat fee for the time the customer has had to spend on the call to BT (most of which of course is online).
Also what about a prize for anyone who can call BT when it doesn't say - sorry we are experiencing an unusually high level of calls at the current time etc. etc. - I never have.