VoIP is highly mobile. Instead of tying a phone number to a location, VoIP can work from any spot with Internet access because completing the connection relies on the IP address of the device, not the station where it is connected. Thus, a VoIP desktop phone moved from one office to another will work, assuming that the network infrastructure is sufficient.

This means that it is not always easy to tell where somebody making a VoIP call actually is. This confusion can lead to semi-comic or tragic results.

VoIP has always had a difficulty reconciling its 911 responsibilities. In the early days, the  main problem was that power failures crashed the phones and the ability to call 911. That isn't a problem with legacy phone systems that are powered through the network and keep operating as long as the phone company's power flows. The problem has faded because the proliferation of cell phones gives subscribers a viable option during power failures.

911 still has its sticky issues, however. The FCC, which issued its ruling on implementing VoIP 911 this week, is grappling with how to provide mobile 911 services. The rules are mandated under the New and Emerging Technologies 911 Improvement Act of 2008. Commissioner Kevin Martin released a statement saying he was "troubled" that wireless VoIP users may lack 911 service if they roam outside their home network because there is no guarantee that the network operator will have data on the "last known cell" location of the caller.

Commissioner Michael Copps released a statement suggesting that it is prudent to explore alternatives other than "last known cell" location for confronting the problem. Significantly, however, he did suggest that no problem exists.

This is a very readable account of all the things that can go wrong with VoIP-based 911 service. Some highlights: There are potential problems with home wiring, home power failures, a broader power failure, and carrier problems unrelated to power. There also are potential problems with E911 providers, which are separate entities. The bottom line is that there more things that can wrong VoIP than with traditional phone services. Legacy phone companies have a simpler distribution model and decades more experience in provisioning emergency services.

Last week, I interviewed Todd Young, the vice president of marketing for Rosum. He pointed to potential 911 issues related to the use of femtocells, which is expected to increase over time. Femtocells can't operate without Global Positioning System (GPS) signals for timing and synchronization. These are fragile signals that struggle in urban and interior environments.

911 is a key issue in the evolution of VoIP. It is likely a side concern to large enterprises. However, small and medium-sized businesses (SMBs) and, especially, small office/home office (SOHO) operations likely to combine residential and business phones should keep it in mind as they make their plans.