Orange, a unit of France Telecom said the new service would be aimed at existing and new Orange mobile business customers and was part of its strategy of becoming a "total communications provider".
Orange said it would charge up to 20 percent less than BT for its service and would offer customers a range of voice "bundles" that could cater to specific mobile, national and international needs.
"By bringing together mobile and fixed services, total spending for the customer will be reduced and, by working with just one supplier, the supply chain and service support models will be simplified," Alastair MacLeod, vice president for business solutions at Orange UK, said in a statement.
Orange, which plans to use the network of BT's rival Cable & Wireless for the service, said customers would be offered a single bill for the service, unlike in the past when they had to pay one company for calls and BT for line rentals.
Orange said the service would offer consumers discounted rates for calls between Orange landlines and Orange mobile numbers, as well as for calls to 10 other numbers nominated by customers.