if u want adsl then u will need a BT phone line, as the signal from the computer goes down the same line, and is filtered at either end. you can have CABLE broadband without a phone line as this works in a totally different way. all depends what suppliers cover your area!
if your with bt youve either got call barring or ocb (outgoin calls barred) . either way you are still payin the same price for the line rental as a two way line holder is and you would be able to get broadband .