I don't know why they can't just use QOS on their own phone network. They could mark the first 2-5 GB of capacity as high priority, and then the rest low priority.
You have just reimplemented tiered data plans, as used in Australia and other parts of the world for DSL. Hit the quota, get shaped to 64/128/256 kbit (depending on ISP and plan). High-quota plans cost more, eg: http://www.internode.on.net/residential/broadband/adsl/extreme/pricing/