VoIP and ENUM

The majority of people use the regular PSTN system. As of now, this is a fact which every VoIP operator has to live with and has design their service around it. Calls from a VoIP phone to another VoIP phone should be free, but if one person is using a regular telephone line, it will be charged. Because of this dependence, a lot of the current set characteristics of VoIP are skewed into an artificial shape. The reality of the dominance of the PSTN system makes it difficult for VoIP to truly cut loose and stand on its own terms.

The backbone of the PSTN system is the telephone number – and its dominance leads even VoIP services to adopt this standard. Therefore, when you ask a VoIP user for their identifier, they don’t give you their SIP address, but the telephone number which maps to it.

At first glance, there’s no way to tell whether a particular number belongs to a VoIP service or is a 100% PSTN number. No special prefix or suffix exists to make this determination. So by default, the telephone companies are involved when a number is dialed. The call goes over the PSTN network and connects to a VoIP phone even though strictly speaking there’s no need to.

The solution here is to have a database which maps every VoIP number to its SIP address. That way, whenever a VoIP user dials a number, it’ll look it up in the ENUM database. If it finds that there’s a corresponding SIP address, the VoIP service can connect to that phone directly over the Internet without going through the PSTN system at all! This will make the call effectively free.

Unfortunately this requires all VoIP companies to voluntarily contribute the records of their customers to this ENUM database. Without that, a query to the database for a VoIP number belonging to a large provider will not return anything and the call has to proceed over the PSTN system. Now the biggest VoIP providers in the US today are telecom companies who are keen to protect their revenue streams. It’s not in their interests to contribute to the ENUM directory and allow calls to be made to their customers for free. It’s a conflict of interest and without their participation, the ENUM directory will be woefully incomplete.

Luckily smaller VoIP companies are increasing in number and are contributing the needed records. Hopefully one day the database will be large enough to reach a critical mass and act as a significant benefit to VoIP customers which will then force the other telecom companies to play nice as well. But till that happens, VoIP will remain crippled and won’t realize its full potential.