T-Mobile USA is targeting the enterprise sector with devices and service that can switch between Wi-Fi and GSM.

T-Mobile Wi-Fi Calling with MobileOffice is a new service offering that enables organizations to extend desktop phone functionality to mobile devices, the company says. The service uses the BlackBerry Mobile Voice System to mobilize PBX systems and initially is launching with six BlackBerry devices.

Unlike rivals Verizon Wireless and AT&T that have ties to wireline companies, T-Mobile doesn't have a vested interest in wireline, so displacing or replacing wired phones works to its advantage.

The service provides for one phone number and one voice mail box. Such a system lowers costs for the enterprise because they don't need to spend additional money on handsets for the desktop, and it increases productivity because employees don't have to miss calls or check voice mail at multiple places, according to John Horn, national director, T-Mobile USA.

If a company has at least 100 lines, they get Wi-Fi calling for free; companies with fewer than 100 lines will pay $9.99 per line each month. "The savings can be pretty substantial," Horn says.

Products available with T-Mobile's Wi-Fi Calling with MobileOffice include: BlackBerry Curve 8520, BlackBerry Curve 8900, BlackBerry 8820, BlackBerry Pearl 8120, BlackBerry Curve 8320 and BlackBerry Pearl Flip 8220.