999 calls were also unable to be made via landline, but remained possible on mobile devices.
At the time, the Jersey Competition Regulatory Authority (JCRA) had been completing a separate investigation into system failures with the 999 service provided by JT and Sure earlier this year.
Pictured: Broadband services were affected across the island during the outage.
The JCRA has since decided to withhold the publication of its initial findings in favour of contracting telecoms specialists Cognitio Consultants to conduct a full probe into the technical issue, while also investigating the company’s governance and security.
As well as establishing what went wrong, Cognitio will be making a number of recommendations and suggesting what sanction, if any, should apply.
The investigation is expected to take two to four weeks.
It is unclear how much it will cost, but the authority said it expects JT to bear the full costs.