An industry is quietly forming around carriers’ use of ENUM, a standard protocol for resolving phone numbers into IP addresses. Originally developed to link consumers' phone numbers to various IP services, the protocol is instead proving useful for interconnecting carriers’ voice-over-IP (VOIP) networks.

The so-called carrier ENUM business is poised to take off in parallel with VOIP services. That’s because ENUM lets carriers interconnect VOIP networks directly and avoid access fees for transmitting calls over the public switched telephone network (PSTN) (see Cable Cadre Talks VOIP).

Consider the following hypothetical scenario. A user of Vonage Holdings Corp.’s VOIP service calls a user of 8x8 Inc.’s VOIP service, and the call must travel over AT&T Corp.’s (NYSE: T)PSTN network to get from one VOIP network to the other. As a result, Vonage incurs a fee. But if 8x8 puts its customers’ phone numbers in an ENUM registry, the call from Vonage can trigger a query to the registry, find the target phone number, and connect directly to 8x8’s network without crossing the PSTN.

The owner of that registry stands to make money from such interconnections. Just ask VeriSign Inc. (Nasdaq: VRSN), which has made millions of dollars from its Domain Name System (DNS) business. On May 3, the company introduced MSO-IP Connect, a service that uses the ENUM and Call Management Server Signaling (CMSS) protocols to route VOIP calls between cable operators and other VOIP carriers through a secure system (see VeriSign Vamps Up VOIP). VeriSign charges an annual fee for the service. The company is developing ENUM registries to interconnect IP-PBX systems and wireless networks, too.

“There are islands of VOIP,” says Tom Kershaw, VP of next-generation networks atVeriSign. “And those islands need a trusted third party to connect with each other. When you default back to the PSTN, a lot of the features of VOIP don’t work, like you can’t do video or collaborative workflow. So, having an end-to-end VOIP connection is critical to enabling the service suite.”

Stealth Communications Inc., a New York-based ISP, in April added an ENUM registry to its Voice Peering Fabric (VPF), an exchange that lets VOIP carriers buy and sell minutes (see ENUM Registry Launches). With the new service, VOIP carriers can not only buy the right to route calls over each other’s networks, but also look up numbers on each other’s networks to interconnect calls. “There are about 1.2 million numbers in our ENUM registry,” says Shrihari Pandit, CEO of Stealth Communications, which charges a monthly fee for use of the VPF.

Some carriers are opting to run their own ENUM registries, and Lowell, Mass.-based NetNumber Inc. sells software that lets them do it. “Our vision has always been to use the basic ENUM standard to create a standardized base of products that can be used by operators and carriers internally for their own use or for use between carriers,” says Glenn Marschel, CEO of NetNumber.

Some of NetNumber’s largest sales have been to wireless operators that use ENUM to look up local number portability (LNP) information in other carriers’ databases. The protocol is also useful for transmitting Multimedia Messaging Service (MMS) messages from one wireless network to another.

Such uses of ENUM stray far from the original purpose of the protocol, which was developed to enable public registries in which anyone could list their ENUM address and phone numbers (see Internet Gets Phone Numbers). By registering, people could establish a single address that could route calls to multiple VOIP phone numbers or IP addresses.

Standards bodies, government regulators, and industry consortiums have been testing public ENUM registries around the world for five years, but demand for such services has yet to materialize (see N. Americans Plan ENUM Directory). “No one has figured out why a user would want to register,” says Marschel at NetNumber. Though public ENUM efforts continue, carrier ENUM is on track to outpace it.

(Share your views on the Enum Phenum. Take our June Research Poll: ENUM: Vital for VOIP?)

— Justin Hibbard, Senior Editor, Light Reading