Haven't come across a complete list of all reserved numbers. In many cases it will depend on what services you are actually using (how many ring groups, etc., that get assigned a number). I seem to recall that I wasn't able to have an extension that began with a zero, but that may have changed in V14. Many new systems just start numbering with 2XXX, 3XXX, 4XXX and so on, saving the upper digits for access to certain trunks. Unless you are deploying massive system, that is 1000 extensions per prefix.