VoIP Guides has an article on ENUM and how it will help propel VoIP further along. It provides a basic description of what ENUM is and why it's a good thing, but it doesn't explain why carriers are reluctant to implement it. For the largest telcos, it eats into their telecom revenue stream. For everyone else, it creates a trust issue. Hit the jump for more.

Let's say, for instance, that Vonage and VoicePulse wanted to connect calls between the two companies without going through the PSTN at all. ENUM just tells you the SIP URL for any given telephone number. Each provider has their own unique SIP URL for each device attached to their network, so this should be easy, right?

Most "voice line replacement" services don't allow customer devices to communicate with anything other than the specified SIP proxy. This is done for the security of the endpoint as well as the security of the service. The provider SIP proxy will relay the necessary communication to either other endpoints or to the PSTN as necessary.

In order for ENUM to work, the providers must do one of two things: either open up their SIP proxy to allow anyone to talk to it-unlikely to happen-or to figure out how to establish peering between all the different providers out there. The peering needs to happen in a way that that maintains the security of the various networks without allowing random participants from other services (or even hackers) from injecting unauthorized calls into the network. Individual providers would either need to peer with each other-a daunting proposition in and of itself-or providers would have to peer with a neutral third party-one they'd presumably have to pay.

This is starting to sound like a replacement PSTN that is likely going to cost about the same as if they just used the PSTN to complete the call to begin with. Why are we going down this road again?

I'd love for someone to tell me that I'm missing something that will make ENUM work. Maybe I have this all wrong. Let me know by leaving a comment.