Coming up to 2 years since I first had an STB (we're talking August 2006), I didn't need a phone then. Guess I was either lucky or things had changed.
Only time I've ever seen them suggest you need to take the phone line is if you want the 'free' channels, that is the box without a subscription. If subscribing to a package no phone line was required for me (we had XL BB + XL TV). 6 months in, however, they rang me and offered me a free phone line and unlimited local/national calls for 5p LESS than I was paying. Seemed odd, but we accepted it (and used it!!) and got billed 5p less as offered.
Currently only have BB, but move into a new property in a few months where hopefully I'll get XL BB + XL TV again.