The expensive part of running a cell phone network involves ensuring coverage and capacity. Anything that keeps customers off the network is a good thing from this perspective, and companies have been experimenting with ways to get users to send their calls through their own broadband connections. Last year, T-Mobile started offering the HotSpot @Home service, which gave users unlimited VoIP calls from home with a phone that can seamlessly hop onto cell networks only as needed. Now, a number of other cell providers are experimenting with an alternative: femtocells that plug into home networks.

From the network's perspective, femtocells accomplish the same thing as an IP-enabled cell phone, in that they shift…