British Telecom (BT) says it has repaired more than half of the phone lines knocked out by a fire beneath Manchester city centre on Monday.
The fire caused 130,000 phone lines to be knocked out
BT said it had reconnected 72,000 lines but still had about 58,000 to repair.
The fire, which caused disruption across the North West, was probably caused by an electrical fault.
BT said it could take up until the end of the week to reconnect all the affected phones. The fire has cost businesses an estimated £10m so far.
Director of Broadband Services for BT, Andrew Jones, said fire stops inside the tunnel had helped to limit the damage.
He added that the investigation being carried out would include a review of fire precautions.
Manchester Chamber of Commerce said many firms may get as little as £50 compensation.
It has been contacting businesses by mobile phone to try to calculate how much trade has been lost.
Spokeswoman Angie Robinson said: "Businesses in this area are losing £4.5m a day. It's highly unlikely that they will be able to claim any of this back in terms of compensation."
Pauline Quayle, from the Insurers and Investment Bankers Association, said: "I think there'll be a limited amount of recompense from BT."