I have a passionate hatred of "Fair Usage" agreements. As many of my UK friends will know, it's become common there to have an "Unlimited" internet service, that has a usage agreement limiting you to something like 30GB/month. If, like me, you download operating systems, games and streaming video on a regular basis, it's pretty easy to cap that in a week.
Skype are now offering "unlimited" subscriptions - sign up, pay $3/month and you can call anywhere in the US or Canada, including cell phones for free. That's much better than long-distance rates, although not as good as local rates (local calls here are free already).
However, I just found this page: http://www.skype.com/legal/terms/fair_usage/
It's pretty fair - I mean, you'd have to be using the phone a LOT to hit it. But the point is this: If there is a LIMIT to the amount your customer can use your service, then they do not have an UNLIMITED account.
Quit lying to us and just tell us the limits. If I'm looking at the options and I see a 50GB limit compared to a 30GB limit, I'll be impressed and go with the better limit. If you say you're unlimited, but you aren't, you're being deliberately misleading and a pain in the arse.