BT is to offer mobile phones that will prevent users from making GSM calls in the office, by diverting them automatically across Bluetooth to the fixed network.

Those close to the "Bluephone" plan, scheduled to be launched in 2004, predict a huge marketing campaign for the scheme, which gives the user one handset and saves at least 20p per minute whenever a call is made in the office.

The Bluephone project, added to the £5 million that BT is putting into a joint venture with T-Mobile for a combined fixed-mobile package called Mobile Home, shows the company is getting back into mobile in a big way. It's something which its licence now allows it to do and which market conditions make a necessity.

"I suspect that if BT hasn't changed its spots, BT as we know it will probably have been forced to change in ten years' time. It will likely be separate business units, marketed as such," said Matthew Robinson, account director at Norwood Systems, one of BT's partners in the Bluephone trials currently taking place in the UK. Studies show that around 40 percent of calls made in an office are internal; many of these are made on costly mobile phones because people are not at their desks - Bluephone would convert these transparently to free internal calls. The move would simultaneously save customers money and siphon revenue from mobile operators to BT.

The Bluephone trial uses SonyEricsson P900 phones, loaded with the Bluetooth cordless telephony profile. When these are in range (up to 200m) of a Bluetooth cordless phone base station they use that, automatically, instead of the GSM network. Incoming calls are routed to the phone over whichever network is most appropriate.

Marketing is still being put together but phones will appear in retail outlets, around the middle of 2004, with the feature as a free extra (under a different name, since Bluephone is a project name). Enterprise users can take advantage of the feature by buying Bluetooth base stations.

"People want one phone," said Robinson. He acknowledged that there might be issues in marketing a feature designed to cost the mobile service provider money - when phones are sold on the back of a contract with that mobile operator. "BT will put resources into this, even if it doesn't happen overnight."

For domestic users, BT is already sharing revenue with T-Mobile for its Mobile Home plan, which combines a mobile phone with a BT fixed line and which will have a major high-street advertising campaign between now and Christmas. . Bluephone is strategic enough to have been mentioned by Pierre Danon at BT's quarterly results meeting and the trial is being promoted on BT's website already, as The Everywhere Phone