I think there might be some confusion here between a domestic "landline" - i.e. cable or ADSL - and a commercial landline - which may well be a dedicated fibre. Some domestic services are truly unlimited (but often, in the UK at least, with a "fair usage" policy that means they can restrict you at will), and many commercial services are actually changed by the amount of traffic, and so are not unlimited at a flat charging rate, as a domestic link might be.