A recent survey by Which? found that many businesses advertise free 0800 numbers for new customers. Existing customers (and probably those who call to complain) are normally directed to telephone numbers with expensive charges usually an 0845 prefix or an 0870 prefix; in some cases these numbers even earn commission for the company you are calling!
It really doesn’t make sense. In any good business it is important that you look after your existing clients. I am not saying you should not look after potential clients but it cannot be good business to favour new clients over existing ones as it costs 6 times more to attract a new customer than it does to keep an old one.