Illustration: Alex Nabaum

There are about 1 billion fixed telephone lines and 2 billion cellphones in the world. Most calls still travel across traditional systems based on proprietary software and hardware. Soon, though, they will move to networks based on open protocols—known as voice over Internet Protocol (VoIP).

The same thing is happening all around the world—in some cases very quickly. Last winter, the chief technology officer at BT Group PLC, in London, said that during the next three to five years, BT would like to "turn off the public switched telephone network."

If that happened all over the world overnight, and all those phones connected to the Internet instead, it would double or triple the number of objects joined by that network of networks, wreaking havoc on some parts of it. Done right, though, over the next several years, the move will open doors to new telephone services that will make all our lives easier and better.

Today it is by no means obvious how best to call a friend or colleague. Should I call your home, your office, or your cellphone? Should I send an instant message to your cellphone or a text message to your computer? If you're not home, should I leave you an e-mail or voice-mail message? In the ideal world, I would simply let the network figure out how best to get to you.

To realize this ideal we must master the problem of signaling, that is, keeping track of all the potential communicating parties, their equipment, and their services, and selecting the right combination for each contact. As we shall see, there are two technical issues. First, we must merge the telephone system with the Internet through VoIP; this trend is already under way in many companies. Then we must strengthen the portion of the network that handles signaling across the entire world and the Internet, so it can stand up to the resulting load. That task, in fact, is the brief of my company, Nominum Inc., in Redwood City, Calif., which provides software for the domain name servers that lie at the heart of the Internet.

The traditional telephone networks and the Internet handle signaling very differently. Voice customers use traditional phone numbers, while the Internet is based on domain names, such as my company's "nominum.com," and numerical Internet addresses, such as "81.200.68.193." Voice callers will want to keep their phone numbers, even as the calls move to the Internet. At the same time, Internet users will want to place calls by clicking a browser link or perhaps by selecting a menu item on a PlayStation or other game machine. They'll also want to start phonelike conversations and Internet chat sessions, and to use videoconferencing software on their laptops and PDAs. So VoIP will have to support e-mail and Web addresses as well.