Dot-UK domain registry Nominet has just opened the market for ENUM, allowing customers to register phone numbers which can then be mapped directly over the Internet for all-VoIP calling.

Why would you want to do this? Well, if you buy a VoIP telephone service today, there is no central list of which phone numbers are VoIP and which are ordinary analogue lines. So if you call from one VoIP provider to another, chances are your call will have to break out onto the PSTN along the way.

ENUM provides that central registry, so all your VoIP provider has to do is look up the other number in the DNS - and if it's in there, your call can be all-IP, meaning no need to pay for that PSTN leg.

So when I talked to Nominet's marcomms director Phil Kingsland last week, for him it was a no-brainer: ENUM will mean cheaper, better quality calls. He described it as a way to "join up islands of VoIP", much as the world-wide DNS service joins up all the various ISPs.

Of course, it costs money - Kingsland couldn't give exact figures as it'll depend on which ENUM registrar you go through, but he suggested a ballpark figure of £10 to £15 a year, much like registering a domain.

You will have to pay a registration fee for each of your phone numbers, though discounts are promised for blocks of numbers. Then it all relies on your VoIP provider doing the look-ups appropriately.

Wait a moment, though - registering a domain name gives you a hook on which to hang a whole bunch of services, from email and webhosting onwards, but an ENUM registration has but one purpose - Internet telephony. It doesn't even support text messaging yet!

On top of that, no-one knows yet how much it might save you per call-minute - and those savings will only be on your VoIP-to-VoIP calls, not VoIP-to-PSTN, so its cost-effectiveness is uncertain. Oh, and it's primarily a called-party facility - you don't need to be ENUM-registered to use your VoIP phone to call an ENUM subscriber (as far as I can work out, anyway).

So why couldn't the VoIP providers have used the existing DNS and domain system to do the same job for little or no incremental cost? ENUM's challenge was that it needed to reuse today's phone numbers and integrate with the existing phone system, which meant bringing those numbers into the DNS world somehow.

Still, I can't help thinking separate registrations are going to confuse the issue, and that ENUM's turned into a bit of a money-grubbing evolutionary cul-de-sac here. After all, this is infrastructure, not a service.

With a bit of luck, VoIP providers will think along similar lines and will figure out a way to roll ENUM into their services at little or no extra cost to the subscribers.

In a few years time, ENUM really ought to be as comprehensible and relevant to the average phone user as IP addresses are to the average webmail user today, ie. not at all.