Australian telecoms operator Telstra suffered a major network outage today (July 8), which has impacted thousands of its mobile customers.The outage, which started at 4:30 am local time, may have also impacted the country's Triple Zero service which connects emergency calls, while the outage also led to train services being cancelled.Michael Ackland, chief financial officer and group executive, strategy & finance, Telstra, said the outage was not caused by a cyber attack, but likely a "software defect," in a post where the company apologized for the outage."Our investigation into the root cause is still underway, but we're confident we've identified a software defect and have been able to isolate it. We'll share further technical detail once the investigation is complete. This was not the result of a cyber incident," he said.The software defects were related to time-keeping servers at data centers in Sydney and Melbourne.It's the latest major outage suffered by an Australian telecoms operator, following Optus' major Triple Zero outage in September.Although services have since been restored, Telstra is facing questions over today's outage, which lasted for around 12 hours.The scale of the disruption is not fully known. Telstra has more than 11 million customers in Australia, though in an update at around 9:30 am AEST, Ackland noted that most calls and data were working as normal.He also addressed concerns around the operator's ability to connect with Triple Zero (000), Australia's main emergency service phone number."We monitored Triple Zero calls very closely throughout today's outage. We tested Triple Zero calling early this morning and confirmed it was working successfully," said Ackland.Ackland explained that the company has carried out welfare checks whenever a call to Triple Zero is unsuccessful or drops out after connecting. In total, Telstra responded to 395 welfare checks by 5:30 pm AEST, confirmed Ackland."The volume of welfare checks was higher than we expected, which prompted us to investigate further. In some cases, a phone wasn't able to connect correctly on its first call attempt, which triggered a callback from us. In other cases, the call was picked up by the TPG or Optus networks and connected successfully," he added."We expect these numbers are inflated by a larger number of people calling to test Triple Zero, and as a flow-on from the core issue. Our backup systems worked as they should throughout."Australia's communications minister, Anika Wells, hit out at those making test calls to the country's emergency services number, including shadow communications minister, Sarah Henderson, after the Liberal senator said she had “tested” the system, reported by The Guardian.Australia's second-largest telco Optus suffered several high-profile outages in the latter half of last year, including a major Triple Zero outage in September, during which time four people died and 600 people were impacted. Optus was also fined AU$12 million (US$8.3m) for an outage in 2023.