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."
First Jersey oyster shipment arrives through Granville facility
Storm Austen: Clean-up continues
Storm Austen: All you need to know
Haut Du Mont: Jury fails to reach verdict on third gas worker
Storm Austen: Government schools to close on Friday
Haut du Mont: Gas workers convicted of manslaughter
Campaign encourages potential child sex offenders to seek help
Electric car charger grant to end in December
