It seems unlikely that anyone will be able to buy service as a communications provider in the same fine-grained quantities in which a subscriber can buy it. I don't know how pricing is generally structured for ISPs, but I would guess that if you're buying service as a communications provider you are assumed to have at least x number of customers, and will be charged accordingly.
"It seems unlikely that anyone will be able to buy service as a communications provider in the same fine-grained quantities in which a subscriber can buy it"
It is simple. Assuming the Ts&Cs don't define you as a subscriber, you simply write an official letter to your ISP stating that you are purchasing their service as a communications provider and not a subscriber and that upon receipt of the letter they acknowledge their notification of this.
Pop it in the post with registered mail (so you have proof they received it) and you are done.
Whether it stands up in court, though, is another thing...
Cute idea, though.