Disruption to the Telstra phone network this morning also stopped train services in the NSW Southern Highlands.

The telecommunication company's social media platforms were flooded with enquiries from frustrated customers this morning who were unable to send or receive calls.

From approximately 1.30am, some calls were answered with a pre-recorded message saying the area being called is "congested".

Services were restored by 6.30am.

A Transport Management Centre spokeswoman said train services were closed on the Southern Highlands line due to an "external telecommunications fault", believed to be the Telstra outage.

A Telstra spokesman earlier confirmed to 9news.com.au a technical issue was impacting the mobile and internet network in NSW.

"Some ADSL and phone services and some mobile calls are impacted," it said.