BT has confirmed its return to the mobile market with the launch of a new service for smaller businesses that integrates staff mobiles with land-lines.BT said its new One Phone service would bring together a company's office phone system and mobile phone needs into a single system.The group said the service works with a full range of mobile phones by inserting a BT One Phone SIM card and will combine with 4G and unlimited access to more than five million BT Wi-fi hotspots.BT is also thought to be preparing to roll out a mobile product for consumers.BT, which spun off its BT Cellnet business in 2001, has long wanted to launch a mobile service to make up for falling land-line demand, but has so far piggy-backed on the networks of rival operators.Last year, BT bought some 4G spectrum from regulator Ofcom tailored for use in homes and buildings, which it plans to use for the new service.It has run a fledgling service for businesses and its own staff using Vodafone's network but is replacing that with a 4G-capable service from Orange and T-Mobile group EE.BT Business Chief Executive Graham Sutherland said: "Today's announcement, combined with the upcoming launch of our 4G services, demonstrates our ongoing commitment to the increasingly mobile UK workforce."Shares in BT fell 6.7p or 1.7% to 380.4p at 12:06 in London.PW