VoIP convergence used to mean a bunch of softswitches, media gateways, replacing Class 5 switches and - not accidentally - spending telecom dollars that used to be spent on traditional TDM on VoIP voice instead. Not much of that ever happened. Today, carrier planners at all levels are focusing on a new kind of convergence - fixed-mobile convergence (FMC). This time may be the charm.

The idea behind FMC is to create a super-voice service that lets customers mingle calls between fixed-line and mobile handsets. A customer could set up rules that govern the conditions under which a call placed to one or the other line is completed. The easiest example is a rule that says, "If my mobile phone is off, ring the call on my fixed line instead of going to voice mail." This could be expanded to include a test for specific numbers, letting non-critical calls go to voice mail. It would also be possible to do the opposite: Ring fixed-line calls through automatically to the mobile number if the mobile phone is on.

There is clearly a customer value for all of this, but the value proposition for service providers goes beyond making customers happy. Where the value is found varies depending upon what kind of voice carrier is looking at FMC.

An incumbent local exchange carrier (LEC) could see FMC as a way to futureproof wireline voice services against broadband-based VoIP offerings. Most incumbent LECs (ILEC) also have wireless subsidiaries, and a VoIP offering based on FMC ties the popular mobile service to the increasingly price-pressured wireline voice. Because modern 3G services are based on Session Initiation Protocol calling, just like most VoIP services (except Skype), FMC would allow for VoIP-to-mobile calls without going through a relatively expensive public switched telephone network gateway. This improves the economics of IP voice.

For the cable companies, an ILEC drive to FMC creates competitive pressure to follow suit, and they have been signing mobile virtual network operator (MVNO) deals with wireless carriers to create their own FMC services. But cable companies have another reason to like FMC, and that relates to the dual-mode Wi-Fi/mobile handset.

A dual-mode handset is capable of using Wi-Fi in the home (or, in theory, in other hot spots) and standard cellular mobile frequencies when on the road. With the right carrier equipment behind it, such a handset can let a user roam between Wi-Fi and cellular while keeping a call connected. When the user is at home (or at work, for a business version of the service), the calls can be rung through the VoIP and Wi-Fi connection, so there would be no airtime charge.