DID Numbers

How do DID numbers work?
Would it be possible to get a block of numbers from the CNA (Canadian Number Administration) to start VoIP service?
Or do I need to purchase DIDs from a company who sells them.

In Australia, it is my understanding that DID are numbers provisioned by tier 1 telcos. Usually in very large blocks 10,000+ if not more…

In Southern California SBC (for example) allows you to purchase DID’s in smaller quantities, I know a block of I think 10 or 20#'s costs something like $5-8/mo for the whole block.