Mobile phone firm Three has been fined £1.9m for weaknesses in its system that is supposed to ensure anyone can make emergency calls at any time.
A loss of service in 2016 revealed that uninterrupted access to emergency calls was vulnerable to a single technical problem, regulator Ofcom said.
Its investigation concluded that Three did not act recklessly, but the issue could be reasonably be avoided.
The company said it had strengthened its systems.
Gaucho Rasmussen, Ofcom's enforcement and investigations director, said: "Telephone access to the emergency services is extremely important, because failures can have serious consequences for people's safety and wellbeing.
"This fine serves as a clear warning to the wider telecoms industry. Providers must take all necessary steps to ensure uninterrupted access to emergency services."
In a statement, Three said: "Ofcom identified this vulnerability when investigating a separate, unprecedented and unforeseeable October 2016 fibre break outage on Three's network. This resulted in a temporary loss of emergency call services affecting some customers. Three took immediate action and the issue was quickly resolved.
"Ofcom recognises that the circumstances surrounding the October 2016 fibre break outage were exceptional and outside of Three's control."