For those who have played around with the combination of Wi-Fi and voice calling known as UMA, the idea behind Republic Wireless is no different from the Unlicensed Mobile Access (UMA)-based service bundled in some T-Mobile BlackBerry (s rimm) devices. T-Mobile also has UMA available on some Android phones. When inside the office or your home or inside a Wi-Fi hot-spot, all phone calls and text messages are sent and received via the Internet. When there is no Wi-Fi, the calls are routed over a cellular network.

It is not clear if the plans would include 3G wireless data access or if it will cost extra per month.

Is cheap enough?

Republic is doing pretty much the same. The company, however, says it has built its own soup-to-nuts solution to offer the hybrid calling functionality. The company is going to buy wholesale minutes from third-party carriers such as Sprint (s s). Bandwidth.com could easily use other wireless carriers as its wholesale partners. The company says the monthly plan would include unlimited 3G data without any bandwidth caps. Typically, phone companies call 5 gigabytes download as “unlimited.”And like Kineto Wireless’ UMA that is used by T-Mobile, Republic requires you to buy a special phone that can handle this hybrid calling. The company has built this hardware based on Google’s Android OS (S GOOG). The screenshots below show how the end-customer can find out if they are on Wi-Fi or on the cellular network.

As a long-standing fan of UMA, I like this idea of having one number automatically switching between Wi-Fi and cellular networks. It’s also attractive to those who travel internationally and want to save on calling back to the U.S.

However, I can’t get past the need for a special hardware. That need for special client hardware was always a problem for UMA. From the pricing, my best guess is that Republic is going after customers on a tight budget, and in order to attract this set of customers, the company is going to find a way to subsidize the hardware that will increase its total customer acquisition costs, which in turn means longer pay-off time for these customers.

Still, the idea of unlimited 3G data with the service for $19 a month is interesting enough for me to consider getting an additional line.