BT said the fire was caused by damaged electrical fittings, following an upgrade carried out in the Manchester tunnel.
Although BT is refusing to accept liability for the fire, it plans to pay compensation depending on individual contracts and whether "actual loss" can be proved.
Businesses were unable to make calls or use fax and e-mail for days after the fire, which cut about 130,000 lines. The Manchester Chamber of Commerce estimated the total cost to businesses to be £4.5m a day.
The Communications Management Association welcomed BT's move, but called for guarantees on future service level agreements.
David Harrington, CMA director of regulatory affairs, said, "The only reservation I have is that if customers stipulate diversity of routing within their service level agreement, they have a right to expect that all identifiable, potential single points of failure have infact been identified, and the risk effectively removed."