The 8xx system is the one which maps to domain names, not the standard land-line system.
In the United States, due to number portability regulations, the standard land-line phone numbers also map to domain names because they are no longer used for routing calls. In the UK, mobile phone numbers also map to domain names because of regulations that allow you to switch mobile network operators and maintain your phone number.
Perhaps a customer who wanted to make IP addresses "portable" would pay a fee to the ISP whose addresses they are, and maintain redirection equipment to the "real" IPs... And perhaps the price of doing so would actually be higher than just keeping a T1 to that first provider...
There are people who are proposing a mechanism like that in order to do a new type of multihoming in IPv6. http://www.ietf.org/html.charters/multi6-charter.html --Michael Dillon