Optus and Virgin Mobile suffered a two-hour outage to 2G and 3G services in the Sydney CBD on Wednesday morning.
The Optus network, which Virgin utilises, went down for customers from around 9am to 11am.
Optus attributed the outage to a degraded radio network controller which served a number of sites in the Sydney central business district.
Issues with its Canberra Centre mobile cell tower meant Optus customers in the ACT also experienced service interruptions.
The fault caused "intermittent disruption to voice calls and 3G internet access," an Optus spokesperson said.
This is the second major outage for the telco in just over a month. In early September Optus experienced a large scale outage to its IP services for over three hours, which it attributed to a server issue.
The Optus website, online payments, corporate VPN and IP services, external proxies and internet access for Optus stores were all affected that time. Mobile services were not.