requires the phone company to be all digital, the path between your phone company and your ISP to be all digital, and finally your ISP must have a digital phone line. The ISP uses special modems to support 56K - that's why you can never get 56K if you direct connect to a friends modem. The only analog conversion allowed for 56K to work is from you to your phone company.
A single analog system between you and your ISP will cause it to drop to 33.6K standards. I cannot get 56K where I live because my phone company has older analog equipment. I'm not concerned though, I've got a cable modem :-)