Australian mobile carrier Optus has blamed vendor Ericsson for its Triple Zero outage earlier this week.

The outage impacted the telco on Sunday (September 28), with Optus noting that the incident centered around a mobile phone tower site in the Dapto area in New South Wales. 

The outage lasted for around nine hours for people attempting to connect to the Triple Zero network. Triple Zero (000) is Australia's main emergency service phone number.

Singtel-owned Optus said that the Triple Zero outage impacted 12 users.

However, significantly for Optus, Sunday's incident, although confined to a small area, was the second recent Triple Zero outage for the carrier.

The company is already being investigated for the September 18 outage, during which four people died and more than 600 customers were impacted.

As such, Optus has placed the blame for Sunday's outage in Dapto on Ericsson.

"We continue to work closely with our technical partner Ericsson, whose equipment did not appear to operate as it should, to understand the root cause of the issue on a single mobile tower in the suburb of Dapto in NSW," said Optus yesterday (September 30).

Optus stated that calls attached to the tower were not transferred to other networks. The telco said that assessments found that while 5G services were operational at all times, 4G services were not, which prevented calls from connecting. Optus’ 5G does not currently carry voice services in Australia.

"Optus’ ability to detect the outage was impacted as the Ericsson equipment in the cell tower did not alarm that 4G services were not operational," added Optus. "At our request, Ericsson has undertaken a full health review of their elements of our network. This review found that what has occurred on this cell site is an anomaly they have not seen elsewhere."

Optus did note that the tower impacted during Sunday's outage is fully operational, and reiterated that the issue was unrelated to the outage on September 18. 

In a statement to DCD, Ericsson has confirmed it's working with Optus to investigate the issue.

"Based on our current monitoring, we have ascertained that this issue was limited to this particular cell site and that what has occurred on this cell site is an anomaly," said Ericsson.

"We are taking this issue seriously, and it has the full attention of our Australian and global teams. Ensuring the resilience and performance of our mobile network solutions for our customers remains our highest priority."

Following the outage on September 18, Australia's federal government told the carrier it can expect major financial penalties.

Australia's communications minister Anika Wells said Optus “failed the Australian people," while Prime Minister Anthony Albanese said Optus’s behavior was “completely unacceptable."

Optus stated that the outage was caused by a technical failure that occurred during a network upgrade.

"Early investigations show that it appears that established processes were not followed," said Stephen Rue, Optus CEO, last month. "In order to establish why this occurred, we are speaking with the individuals involved. As to the full technical detail of the network failure, we will need to leave that for the investigation."

It's not the first time Optus has suffered a significant outage that has impacted Triple Zero services.

Last year, the carrier was slapped with an AU$12 million (US$7.93m) fine for an outage in 2023. The outage, which happened on November 8, 2023, lasted more than 14 hours.