After more than a year of leaked news and trials, AT&T will ship its femtocell in April: We've heard about and seen pictures of the 3G MicroCell for quite a while. Like all femtocells, the idea is to connect mobile users voice and data calls in their homes or small offices to a broadband connection, improving quality and throughput without taxing AT&T's network. The benefit to customers is better coverage, fewer dropped calls, higher consistent data throughput, and, optionally, unlimited calling.

For carriers, every call or bit of data that they don't have to pass over their expensive, congested mobile networks saves them real money in preventing customer defection and deferred capital expense, while increasing subscriber revenues.

The AT&T 3G Microcell is unique in working only with 3G; Verizon and Sprint's options are 2G only, which allow them to offload voice but not data. For smartphones that have Wi-Fi built in, that's not such a big deal for any of those three carriers, but for customers without Wi-Fi (there still are some) or phones that can use 3G mobile broadband only, AT&T has a much bigger win.

AT&T, like Sprint and T-Mobile (which uses Wi-Fi with the UMA standard), will also offer an unlimited calling plan for domestic US calls placed and received when in range of the 3G MicroCell. AT&T will charge $20/mo for either individual or family plans. That's steep for an individual: AT&T's general mobile plans are $40 for a 450-minute plan, $60 for 900 minutes, and $70 for unlimited calling anywhere. For families, it's a far better deal.

T-Mobile's UMA unlimited calling service is $10 per month (individual or family), and Sprint's Airave is $10/mo/account or $20/mo for a family plan.

The 3G MicroCell will cost $150, with a $100 mail-in rebate if you purchase monthly unlimited service at the same time, and another $50 rebate for those who sign up for AT&T fixed broadband services (DSL or fiber). Sprint charges $100 for its femtocell and a $10/mo fee, while T-Mobile's router is about $50. (T-Mobile seems to have removed pricing information from its site, so I can't confirm at this moment.)

Verizon has no calling plan, but sells its Wireless Network Extender for $250, with no recurring fees. It's meant to improve signal coverage only, which still seems strange to me. I suppose those who want Verizon service and can't get a good signal at home have an option with this device, as opposed to changing carriers.

Phone numbers need to be registered with the 3G MicroCell to be used. AT&T says the 3G MicroCell will be available in mid-April.