The Scottish Ambulance Service was left unable to receive 999 calls for several hours because of a problem with its telephone lines, the BBC has learned.
The incident, on 21 July, meant no emergency calls were received by the service's three Scottish call centres for some hours.
Callers were diverted to Belfast and the north of England.
Ambulance bosses and BT said calls were answered and patients were unaffected. Scottish Labour called for a probe.
BT provides the systems used by the Scottish Ambulance Service to answer 999 calls at its centres in Inverness, Edinburgh and Glasgow.
But, on the date in question, a technical fault in the switch at Glasgow had a knock-on effect on calls to the centres.
The problem first affected the Inverness call centre at 0100 BST, hitting Edinburgh at 0942 BST and resulted in Glasgow only able to receive a limited number of calls from 0956 BST.
Following action, all three centres were fully operational again by 1530 BST.
A BT spokesman, said: "A buddy system is in place in the event of any such failure and the contingency plan kicked in immediately, with calls answered by operators at the Northern Ireland and north west ambulance service as required.
"No calls were lost and service to members of the public was unaffected as all calls were answered."
The spokesman added: "The Glasgow switch was rebooted and specialist engineers sent to all of the sites, with normal service resumed across the sites by early evening."
'Not a repeat'
An ambulance service spokesman, said: "It is reassuring that contingency plans developed for such scenarios worked with the minimum disruption for patients.
"We have reviewed the incident with BT and they have agreed to an action plan to ensure that there is not a repeat of this type of fault."
Scottish Labour's Jackie Baillie, calling on Health Secretary Nicola Sturgeon to make a statement to MSPs, said: "I am shocked that all three of Scotland's ambulance dispatch centres went down at the same time.
"We need an urgent investigation to establish what went wrong and prevent it happening again."