Telecoms firm, which manages the system, said it had taken three days to contact each of the disconnected callers
The company, which manages the emergency services calls system, said it had taken three days to contact each of the callers who had not been connected after what it described as a complex software issue disrupted the service for several hours from 6.24am on Sunday.
"During the disrupted period on Sunday, we have provisionally identified there was a total of 11,470 unique calls that were unsuccessfully connected to 999," said BT, which published the findings of its internal investigation on Thursday.
"For each of these calls, BT's commitment is to call the customer back and establish whether further help is needed and, if required, connect to the appropriate emergency service."
BT said that if it was not able to recontact the caller then the details would be passed to police to investigate. It said the process of trying to get back to people was completed by 8.16am on Wednesday.
The company did not say how many callers it had failed to contact, or if any of the missed calls had resulted in injury or death.
The telecoms regulator, Ofcom, launched an investigation on Wednesday into the outage, during which police forces, ambulance services and fire and rescue services across the UK asked people not to phone 999. The Department for Science, Innovation and Technology said it would launch its own inquiry into the speed at which BT notified the government after the service went down.
BT said Ofcom had been alerted at 9.05am on Sunday and the government officially by email at 9.45am.
It said that after a failed attempt to transfer calls to a backup system earlier in the morning it had successfully managed to do so for landline and mobile 999 calls by 8.50am, although only with "basic service functionality which meant increased call pickup and call handling time".
It took until 21.29pm on Sunday to move off the backup system and return all emergency and non-emergency call traffic back on to its primary 999 network.
"Our investigations have found a complex software issue that had never been seen through our continuous testing regime," said BT. "It was causing a 'caching' issue which resulted in impacted calls not being routed correctly and the user's call being disconnected."
The company has said that a "robust temporary fix" is now in place and a permanent solution is being tested.
Comment: It seems that a number of serious technical glitches are occurring these days: