Meanwhile, the company’s technical experts continue to investigate how at least 70 customers in the Merewether area whose numbers begin with 4963 were billed for the calls last month.

The possibility that a sophisticated caller ID spoofing scam, which involves using false numbers to make calls to a switchboard, has not been ruled out.

Telstra Hunter Region general manager Chris Cusack said last night it was believed the unauthorised calls occurred during a two- or three-day period in March.

‘‘We will proactively identify and contact all customers who have been impacted and reimburse any incorrect charges,’’ he said.

‘‘We certainly apologise to everyone who has been affected.’’

He refused to speculate on what may have caused the 1900 calls to be billed to the landlines.

‘‘We have the right people in the organisation investigating it and I’m confident we will get it sorted out,’’ he said.

‘‘I have said all along there is a range of reasons for why this has happened.’’

The most common number appearing on the bills is 1900910080. It is administered by the offshore information service provider Cloudtel.com and is used by the online gaming company Surfpin, whose websites include runescape.com.

A company representative who returned the Newcastle Herald’s call to an Australian information number refused to answer questions about where or how the calls to the number were administered.

Caller ID spoofing is the world’s fastest-growing form of telecommunications fraud on landlines and mobile services.

It usually involves using an internet-based server that can make telephone calls with false numbers.

Other servers and switchboards are generally unable to detect that the number is fake. US-based Pindrop Security reported more than 1million caller ID spoofing calls in the second half of 2011, a 52per cent increase on the previous six months.