JT has been fined £90,000 by the competition regulator after a network issue caused an island-wide loss of phone and internet services in July 2020.
As well as the internet being down for anyone using JT's network, it also left islanders unable to make 999 emergency calls for the duration of the outage.
The Jersey Competition Regulatory Authority says the fine could have been substantially more, but it took into account that the telecoms firm has cooperated with its investigation and has already taken steps to address the problems.
JT has vowed to overhaul its network, making improvements to make it more resilient if something was to go wrong in future.
It has to regularly report back to the regulator with updates over the next five years.
Tim Ringsdore is the JCRA's Chief Executive:
"This has been a long and complex investigation following an outage of the JT network that affected all consumers and businesses, including the failure of the essential 999 emergency call service.
"Throughout JT has worked with us to resolve the issues and to put in place a system of checks and balances that will improve its infrastructure, process and procedures and help it to meet best practice going forward."
JT's interim CEO, John Diamond, says the company fully accepts the JCRA's decision and that steps have been taken to ensure a similar outage does not happen again:
"JT accepts the final decision made by the JCRA on this important issue. We take any matters like this exceptionally seriously and have worked closely and productively with the JCRA through the process.
I would like to apologise again to our customers for the disruption, and to thank our teams who have carefully and collaboratively worked through the sequence of events which led to the problem in July 2020, to ensure it will not be repeated."

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