The warning comes after phone lines at an Otago business were hacked and more than $100,000 worth of international calls made.
Black Rock Consulting Ltd director Steve Cogger said a client had two call minder services hacked and calls were forwarded to international numbers.
The calls, which totalled about $110,000, were made within a period of 48-72 hours.
The hacking began minutes after the close of business on Friday, December 20.
Hackers had commonly targeted private automated branch exchange networks (PABX), such as intra-company telephone networks, he said.
But this was the first time Mr Cogger had seen call minder services hacked.
''Anyone with a call minder is vulnerable,'' he said.
Although it was a relatively new method of fraud, it had the potential to become more common if security measures protecting call minder services were not robust, Mr Cogger said.
''People should be proactive in ensuring they have a pin number of a minimum of six digits and change it regularly.''
If people noticed irregular calling activity and unexpected costs on their phone bill, they should immediately contact their provider, he said.
Vodafone head of fraud and security Rhys Metcalfe said the method used to hack call minder services was no different from that used to hack PABX networks.
''Any type of call forwarding service ... that is secured by a pin can be a target, from a small business with one line to a large business with a PABX and multiple lines,'' he said.
''By hacking into voicemail, offenders can set up call forwarding to make large volumes of international calls to high-cost destinations in a very short space of time,'' Mr Metcalfe said.
During the past few months, there had been a ''substantial increase in fraud across telecommunications providers in New Zealand'', he said.
''Until recently, hackers focused on European markets, but changes in regulation over there to combat fraud have effectively encouraged hackers to Australasia.
''New Zealand and Australian telcos are co-operating in an effort to combat this type of fraud.''
Telecom communications manager Julie Wagener said the use of call minder services by hackers was ''certainly not a regular occurrence'', but ''unfortunately it does happen''.
Telecom monitored activity to keep fraud to a minimum and would apply an international toll bar on lines where it suspected fraud, she said.
Users were responsible ''for ensuring appropriate security measures are in place'' and reimbursement was awarded on a case-by-case basis, she said.
Mr Cogger said the issue highlighted the need for all users to take phone security seriously.
