The company has apologised “sincerely” for the problems with the emergency phone service at the weekend.
BT took nearly three hours to alert the government about problems with the 999 emergency phone service, a minister has said.
A full investigation will be launched into the delay after the failure meant many calls were not connected, Viscount Camrose told the House of Lords.
Concerns about the resilience of the 999 service were raised in the House of Lords in an urgent question.
BT, which manages the phone system, apologised “sincerely” for the issues.
The issues began on Sunday morning and continued well into the evening, even after BT switched to a “back-up system”, parliament heard.
Pressed over when the government was made aware of the problem, technology minister Lord Camrose said: “The event that caused caused the platform to go down occurred at 06:30 on Sunday. The government was advised of the event at 09:20, so just under three hours later.
“I understand that they informed the government as quickly as it was practically possible for them to do so. One of the areas they will look into as part of the inquiry is whether that should have been, could have been, faster.”
He said the issue has now been fully resolved, and the service is running as normal.
He added: “A full investigation is under way to understand what caused this problem.”
Labour peer Baroness Anderson of Stoke-on-Trent, who previously sat in the Commons as Ruth Smeeth, called it “an incredibly disconcerting event”.
“Any failure in the system will undermine faith in our emergency provision. We are seemingly very fortunate that there was no major incident.”
The technical fault with the network has led to calls from a former Metropolitan Police chief to introduce joint call-handling for the three emergency services and “remove the cost that BT imposes on the whole system”.
Lord Hogan-Howe, who headed the UK’s largest police force from 2011 to 2017, said: “Isn’t it time we started having joint call-handling?”
“The only reason BT need to take the call is because the ambulance, the fire and police have to take them independently, and you have to make a call to BT to declare which service you require, often at a time you don’t actually know which one you need.
“Why don’t we answer them together? Why don’t we remove the cost that BT imposes on the whole system that appears has not worked very well on this particular occasion?”
At the time of the glitch, BT said its priority was getting the lines “up and running as soon as possible”, and experts were trying to work out the cause.