Normalization and dial plans was one of those topics that took me a awhile to fully understand. I think this is true for many people who are being introduced to the telephony world for the first time by attempting to implement Enterprise Voice with OCS. This article does a good job of explaining the how and why of E.164 (RFC 3966) normalization. Highly recommended read.
http://blogs.technet.com/dougl/archive/2009/06/25/phone-numbers-in-OCS.aspx
