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Understanding Telephone Number Translation

I think of Unified Communications (UC) as expansive as well as contractive. Like a lump of Silly Putty, it can be stretched and squished to meet the ever changing needs of its users.

It wasn't that long ago when electronic communication was limited to telephone calls. You picked up a telephone receiver, pressed a finger down on a series of numbered buttons, and voila, you were able to speak with someone miles away.

Sometime in the 1980s we added email to our bag of communications tricks. Since then we've expanded to video, instant message, WebRTC, and SMS text. I would be remiss if I didn't add Facebook, Pinterest, Reddit, Twitter, and other forms of social media to that list. There are days when I feel I communicate more through these new forms of engagement than anything else.

For the millennial generation, land-line telephones have become passé and cell phones, nay, smart phones are used for nearly anything except voice communication. Those of you with teenage children certainly know that.

As we continue to add new ways to communicate, the address for all these forms of engagement becomes very important. Imagine what it would be like if my VoIP address was [email protected], my video address was [email protected], and [email protected] was used for instant messaging. Remembering all those addresses and how they are applied would create a condition where people stopped using one or more forms of communication.

Instead, UC allows me to condense that list into a single address for voice, video, and instant messages into a single format -- SIP:[email protected]

In the IP world, SIP:[email protected] is known as a Uniform Resource Identifier (URI). "SIP" indicates the protocol for this form of communication, "aprokop" is the subscriber name, and "" is the domain of the subscriber. Put them together and you are findable from anywhere in the world from any IP network.

Of course, can be used for services outside of UC. For example, Web, email, and communications could all be member services of The protocol designation in the URI is used to determine which service should be invoked -- e.g. "SIP:" for unified communications and "mailto:" for email. The important aspect is that the remainder of the URI can stay constant.

This leaves us with a big problem to solve. How do I pick up an old-fashioned, analog telephone at home and call my SIP telephone out there on the public Internet? DTMF touch tones are not easily translated into alphanumeric values. Where's the "@" on a telephone dial pad? Additionally, resources like the White Pages have yet to understand anything but numbers to identity a telephone user. However, as I UC-enable my enterprise, I don't want to prohibit calls from the public switched telephone network (PSTN) to the SIP world.

Enter E.164 Number Mapping, or ENUM for short. ENUM is a way to translate E.164 numbers (the standard format for telephone numbers) to something more Internet friendly. This gives a UC user the ability to employ a user-friendly, character-based SIP name that maps to an IP address that maps to a number that can be dialed from any telephone in the world. ENUM is a collection of proposals (it's best to start with RFC 6116) that are managed by the Internet Engineering Task Force (IETF) -- the same people who oversee SIP and all the Web protocols.

For instance, anyone who dials 952-456-3516 can reach me on the myriad of SIP soft clients I registered to [email protected] The caller has no idea that any number to name translation has taken place. He or she dials a telephone number and ENUM steps in to do the heavy lifting. What could be simpler?

As with nearly every other form of Internet address translation, ENUM utilizes the Domain Name System (DNS) and DNS NAPTR records to perform the mapping between telephone numbers and IP addresses. Specifically, it uses the public domain for ENUM services.

While the actual mapping of telephone numbers to IP addresses is somewhat complicated, the only thing that you need to understand is that in order for a number to be available for translation it needs to be registered with the domain. Once registered, the DNS lookup process will return an IP address for the telephone number query.

Of course, some enterprises might not want their internal IP addresses available to anyone who asks. This is where private ENUM comes in.

Private ENUM acts exactly like public ENUM with the exception that the domain is not used for the telephone number to IP address mapping. Instead, an enterprise will create its own domain that restricts access to only authorized users. An authorized user may be an individual or a trusted organization such as a PSTN carrier.

Over time the privacy issues that arise from public ENUM may be resolved and private ENUM no longer required, but until then, enterprises may wish to maintain their own ENUM system.

It's important to note that both public and private ENUM are still works in progress. In fact, I hesitated to write this article thinking that ENUM might be considered obsolete. It has clearly faded in importance and the excitement that existed a few years ago has waned. Still, PSTN number to IP URI translation is a problem that must be eventually solved.

ENUM is a technique that may or may not be there at the finish line. Adoption has been less than what was hoped for, but it's out there and is being used. Stay tuned as this technology evolves.

Andrew Prokop writes about all things unified communications on his popular blog, SIP Adventures.

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