Well you're paying the £12 for line rental and they automatically put you on a base phone plan. For me it's M which is just weekend calls. You don't have to get a phone though, if you're worried about people ringing up mobiles from it.
Most ISP will do that.
So who are you with then may I ask? Apologies, it's been a long day

I was with Demon due to the unlimited download usage, but their tariff plus the line rental puts its to well over £30
