Optus says it has resolved a "major outage" that left customers of the country's second-largest telecommunications company with limited service for about five hours.
The Optus website shortly after 8:00am AEST advised customers that "voice service may be interrupted" and some people were unable to make calls.
Many people on other networks also reported not being able to reach people on the Optus network.
The company also said it was aware some NBN landline customers might be "intermittently" experiencing issues making calls.
Optus tweeted that the problem was fixed early this afternoon.
"We sincerely apologise to our customers for any inconvenience and thank them for their patience while affected services were restored," it said.
The outage caused confusion of people on landline and other mobile phone networks.
Telstra this morning said it was working with Optus to have the service reconnected, telling its customers "there are no issues with our network".
