But why can't I run Voice Over IP and get free calls over the Internet?
In ten years time you will but at the current moment would you want to use a computer for cruciual telephone calls? How many other users do you know who have a VOIP phone? The idea is OK for corporates with dedicated lease lines and the supporting infrastructure but it's someway off for the majority of telephone users until the price moves into line with established technologies.
If broadband is currently not available in your area and you need a faster Internet connection today then ISDN provides the only viable solution. If you bond the two channels together and invoke data compression you can get pretty close to 256K which is the same as a 512K broadband connection with two users onboard. Don't forget you can still get un-metered single channel Internet access with ISDN for £15 per month.