JT has been fined £380,000 after the island's 999 service was down for 57 minutes for most phones in September last year.
All landlines and mobile phone services from Sure and Airtel were affected, with around half of JT's mobile services.
It happened at 1.45 am on 20 September 2022.
The Jersey Competition Regulatory Authority says JT 'failed to take all reasonable steps' to ensure the integrity of its network.
"The performance and security of Jersey’s public emergency call service is critically important.
"Therefore, in the interests of the safety and security of Jersey’s public, the JCRA determined it necessary and proportionate to issue a financial penalty."
JT is required by law to provide the island's emergency service call service.
The telecoms company says it is working to improve its network.
"JT is part way through several programmes of work which will transform and modernise Jersey’s networks providing best of class resilience and capability."
Phone robber jailed after targeting a 'lone' woman
Channel Islands' parliamentary Standards Commissioner resigns
Island Energy charged over Mont Pinel gas explosion
Jersey airport to open early due to broken scanner
Critically endangered species of lemur joins Jersey Zoo
Lido to 'pause trading' amid ongoing uncertainly
Ports of Jersey applies to remove Gorey Pier landing stage
Jersey's police officers to be held to new standards
