Mobile phone calls received in Japan from nonexistent international phone numbers surged in September in a suspected billing fraud and victimized at least one woman, experts warned Tuesday.

The indiscriminate calls in Chinese language appear to be made by a bot program targeting Chinese residents in Japan, but many Japanese received them, the experts at security technology firm Tobila Systems said, urging people not to take calls from or call back dubious numbers.

Calls from numbers that start with nonexistent country codes such as "+83" and "+422" were first identified around August and more than 5,000 were recorded one day in September among the spam phone numbers Tobila deals with in cooperation with nationwide police, they said.

A Chinese woman in her 40s in Akita Prefecure was forced to pay ¥4.11 million into a bank account after answering a call from a number with nonexistent country code "+885" on Sept. 10, investigative sources said.

During the call, a man and woman claiming to be from the Chinese Embassy threatened the victim and accused her of illegally sending masks to Wuhan, China.

Most of the calls are believed to be automated and have recently started using numbers with fake area codes after initially using codes from countries in North and South America, Tobila said.