Deutsche Telekom's T-Mobile USA plans on Monday to start offering a wireless service for office workers, hoping to increase its presence in the corporate market as it chases larger rivals like AT&T and Verizon.

T-Mobile USA, which focuses now on cost-conscious consumers, said its new service combines BlackBerry phones from Research In Motion with Wi-Fi -- a short-range radio technology -- and would help companies save money both on cellphone calls and calls from desktop phones.

Corporate BlackBerry users could get rid of their desktop phone as their BlackBerries will revert to the Wi-Fi network from cellular within the office -- or anywhere else they have access to a Wi-Fi network.

This means using up fewer cellular minutes and eliminating the need for wired phones, T-Mobile USA said.

Analysts said that, unlike companies such as AT&T, which has to protect its traditional telephone business, T-Mobile USA as a purely wireless operator would not have to worry about cannibalizing fixed-line revenue.

From T-Mobile's perspective, this really launches them into the market, said Rob Arnold, enterprise communications analyst with Current Analysis.

They don't have the disadvantage of trying to protect their revenue streams from fixed-line networks, he said, adding that any sales through the new service would be entirely new revenue.

T-Mobile USA is the fourth-ranked player in a mature U.S. mobile market and is under-represented in the high-end corporate arena.

Analysts say it has to beef up spending considerably to catch up with markets leaders Verizon and AT&T.

The new corporate offering, similar to a residential service already in place, will charge a flat monthly fee on top of cellular rates. The fee, which depends on the number of users signed up, can include unlimited Wi-Fi calls.

T-Mobile also promised uninterrupted service: if a user moves out of a Wi-Fi zone during a conversation, the call will automatically switch to T-Mobile USA's cellular network.

(Reporting by Edwin Chan; Editing by Bernard Orr and Leslie Adler)