enum is a method of matching PSTN numbers to the DNS space.

That's like saying that DNS is in turn just a method for turning hostnames into IP addresses. Sure, that's what it does, but in practice it does a lot more than that - it plays a large part in allowing you to "find" a website.

Similarly ENUM allows you to "find" how to connect to a "PSTN" number (which of course might or might not actually be connected to PSTN).

If I tell you that my phone number is +1 408 500 0600, then without ENUM (or some other similar service) what option are you going to have to call me from your VOIP service? You're going to have to pay for an international call to that number, over the PSTN network and very probably via at least a 3-4 carriers. However if my VOIP provider (not necessarily me personally) had an ENUM record for that number then you could lookup the corresponding SIP address, and instead make a point-to-point connection.

Clearly the end result is the same - my phone rings - but without ENUM or something similar there is no optimization of the path taken, and the call is almost certainly going to leave the TCP network and go through at least 2 SIP-SS7 translations - and with a much higher cost both to you and to me!

I'm not saying ENUM is the right answer, but there's defintiely a problem out there that a technology is required to solve - and ENUM is the best option available at the moment.

Scott.