More than 130,000 telephone lines remain blocked in the North West after a fire started in a BT tunnel in Manchester city centre.
Firefighters initially had difficulty getting to the blaze
Emergency services have been affected, and police say it is a major incident.
Fire crews had to climb down 30 metres and then crawl 150 metres to get to the fire, which began at about 0330 BST.
BT engineers have gained access to the tunnel after it was closed for most of the day. It is not yet known how long it will take to repair the damage.
Telephone lines have also been affected outside Manchester, with problems with Cheshire's 999 service.
Phone lines are also down in Merseyside, Lancashire and north Derbyshire.
Greater Manchester Ambulance Service said it had been put under "extreme pressure" after the blaze damaged its radio network.
The control centre is using mobile phones to contact emergency crews.
An ambulance spokeswoman said: "We send messages out to ambulances via radio, but that is not working at the moment.
"We are extremely busy and we are working tirelessly to get the radio up and running.
"At the moment it is not affecting emergency and 999 calls, but we are extremely busy and the situation is being monitored and extra staff have been
Emergency calls in Cheshire were also affected as 10,000 phone lines in the east of the county went down.
A Cheshire Police spokesman said: "Our ability to receive 999 calls has not been affected but the ability of people using those 10,000 lines to make 999 calls from land lines may have been.
"We would urge them to use mobile phones."
The force also increased patrols in the areas where phone lines were not working.
Assistant divisional officer Ian Bailey, of Greater Manchester Fire Service, said the fire itself was not too serious but reaching it proved problematic.
He said: "The firefighters needed breathing apparatus and were using a lot of air up by the time they reached the fire, so we had a high turnover of firefighters."
Firefighters had to ventilate the tunnel before BT engineers were allowed down.
Freeserve, the internet service provider, said customers in the Manchester area had experienced difficulties with their broadband connections as a result of the fire while AOL said its customers in the area, who are connected to the BT network, would also experience problems.
A BT spokesman said: "We don't know what caused the fire at this stage but it is affecting more than 130,000 homes and businesses in Manchester."
The firm described the damage to the cabling as extensive.
BT said fixing the ambulance radios was the top priority, but could not give a timescale.
Anyone wishing to report a non-urgent incident was asked to wait until full phone service had resumed.
People experiencing problems with their telephones were also asked not to report the fault.