I've had to ring them recently, and it took me over 10 phone calls to get the issue sorted out. I found the best time to ring is weekdays in the morning at about 09:30.
I called SKY at one stage, and they would have been twice as expensive as NTL (we've 3 NTL boxes), and they wouldn't give any discount to me as a (potential) new customer ... remember you may have the same hassle with Sky getting installation etc. done.
If you are having trouble with them, its useful to ask to speak to a supervisor, or get the operator to refer your case to a supervisor. And always take a note of the operators name, and their supervisors name ... once I started doing this, I found that they phoned me back when promised, and their "customer care" improved dramatically!