Optus suffered from a "major" network outage on Friday afternoon affecting thousands of its customers in NSW, Victoria ad Tasmania.
The outage appeared to start at about 3.30pm AEST and ended at 6.30pm AEST.
"We've identified a fault which means you might experience issues with voice at the moment," the telco said on its website.
"Our team are on to it, and aim to fix this as soon as possible. We're sorry for the inconvenience."
Optus reseller Amaysim was also suffering from the outage, alerting its customers via Facebook to the issues.
"Optus mobile services in New South Wales, Victoria and Tasmania have now been restored," an Optus spokesperson said in a statement just before 6.30pm AEST.
"We apologise to customers affected, thank them for their patience and appreciate them getting in contact. We will continue to monitor these services closely."
Optus was last hit by a major outage on its mobile network in June.
Customers on Optus' Facebook page were not pleased about the latest issue.
"Awesome service Optus!!! Can't make work calls. Will u pay for the lost time??" wrote Stef Lesser.
"Your network sucks, why won't my phone ring!!" wrote another customer, Shane Dickinson.
with Kate Aubusson
