The service will tie into office PBX (private branch exchange) systems so employees can use their T-Mobile BlackBerries in the office on Wi-Fi, using the same number as on their desk phones. They will also have one voicemail box and call filtering system for both phones. Companies could even remove their desk phones and simply use the BlackBerry.

Extending PBX features to mobile phones is nothing new, but creating a service offering specifically for businesses is a rare move for T-Mobile. The nation's fourth-largest mobile operator has overwhelmingly marketed to consumers, making its name with devices such as the youth-oriented Sidekick texting phone. The carrier, based in Bellevue, Washington, tends toward prepaid and less-expensive monthly plans than the larger U.S. operators.

But T-Mobile already utilizes Wi-Fi in its HotSpot@Home service, which lets customers make unlimited calls with a Wi-Fi-enabled cell phone over their home wireless networks. Adapting this idea to enterprises could make for a compelling option for cost savings and convenience. The system can save customers money on international long-distance calls and roaming, as well as reduce the consumption of domestic minutes, according to the carrier.

Wi-Fi Calling with MobileOffice works with a wide variety of PBXes and lets administrators apply authentication, call logging and security to employees' use of the BlackBerries in the office, T-Mobile said.

T-Mobile will work out the price of the underlying BlackBerry Mobile Voice System with each customer, according to a representative from the carrier's PR agency. Companies with more than 100 T-Mobile voice and data lines will get free unlimited Wi-Fi calls on BlackBerries nationwide and from abroad to the U.S. Companies with fewer than 100 lines will pay US$9.99 per line, per month.