BT is extending its fixed/mobile Fusion service to small businesses, claiming that UK mobile users can make major savings on calls.

The system allows mobile handsets to make cheaper calls via broadband IP systems in the office when in range of a transmitter. When out of range calls are routed through BT's mobile infrastructure as normal.

The service can only be used with a BT broadband line and the telco's own-brand wireless hub.

"BT achieved an important milestone in fixed/mobile convergence when it launched BT Fusion last year," said Bill Murphy, managing director of BT Business.

"We are now making the cost and convenience benefits available to businesses too, starting with smaller businesses."

BT launched the Fusion service for consumers in June and is expecting strong demand, although no usage figures will be released until later this week.

Analysts have predicted modest interest in the service. "This kind of service is not going to take off just yet," said Rachel Lashford, an analyst at Canalys.

"But it has a great deal of potential for the future, and there is certainly a threat to mobile operators from fixed/mobile services."

Users are currently limited to two phones, Motorola's V560 and Razr V3B, with the latter proving most popular according to BT.

In a promotional offer running until 31 March, BT is offering a free wireless broadband hub and a subscription at £6.25 per month per handset for a 24-month contract.