JT has been fined £380,000 by the telecoms regulator after Jersey residents were unable to make calls, including to 999, for nearly an hour.
The outage happened on Tuesday 20 September 2022 at 1:45am.
The JCRA said JT "failed to take all reasonable steps, within its control, to ensure the integrity of its network".
It said this caused a 57-minute outage affecting all Jersey landlines and most mobile phones, including those on other networks using JT's infrastructure.
The authority added that "the performance and security of Jersey’s public emergency call service is critically important", which is why the fine was given.
"JT is part way through several programmes of work which will transform and modernise Jersey’s networks", a JT spokesperson said.
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