Optus suffered a two-hour outage on its 3G network on Monday evening, leaving customers in parts of Sydney unable to make or receive calls or text messages.
A spokesman for Optus told iTnews that the outage "was caused by a transmission fault that resulted in a call signalling failure".
"It resulted in the loss of some 3G voice services in Sydney CBD while some 2G voice services were impacted in the surrounding areas (eg. Eastern Suburbs, Lower North Shore, Northern Beaches and Sutherland Shire)," the spokesman said.
The carrier had earlier asked customers to "please switch to 2G for calls + SMS" in a tweet.
Users from across Sydney and as far afield as Newcastle reported the outage on the broadband forum Whirlpool.
Services appeared to drop out about 5.30pm before being restored about two hours later.
Optus apologised for the inconvenience caused to customers.