Mobile provider Three has been fined £1.9m ($2.4m) after some customers were not able to phone emergency services.
Regulator Ofcom said "the fine reflects the seriousness of the breach", as being able to connect to 999 services "is of utmost importance to public health and safety".
The mobile watchdog said Three, owned by Asian conglomerate CK Hutchison, experienced a fault on its network last October in parts of London, Kent and Hampshire.
It added that its investigation showed emergency calls from the affected areas had to pass through a particular data centre. This meant Three's emergency call service was vulnerable to a single point of failure.
It said the network should have been able to automatically divert emergency calls via back-up routes in the event of a local outage.
Ofcom enforcement and investigations director Gaucho Rasmussen said: "Telephone access to the emergency services is extremely important, because failures can have serious consequences for people's safety and wellbeing."
"Today's fine serves as a clear warning to the wider telecoms industry. Providers must take all necessary steps to ensure uninterrupted access to emergency services."
The regulator acknowledged that the network operator has since strengthened its operations.
Three said: "Providing our customers with uninterrupted access to emergency services is a requirement we take extremely seriously. Three took immediate action and the issue was quickly resolved."
The operator has 20 working days to hand the fine over to Ofcom, who will then pass the cash onto the Treasury.