Police are investigating after hackers ran up a £4,000 bill by using the phone lines of a Flintshire help group.
Flintshire Local Voluntary Council (FLVC) found its office phone had been used to make calls to overseas numbers in Africa and premium rate lines.
It is now in talks with its phone provider to try to avoid paying the bill.
Det Sgt Andrea Barnard, e-Crime Wales police manager, said steps can be taken to prevent Dial Through Fraud (DTF).
"DTF occurs when hackers hack into switchboard systems and exploit their vulnerabilities," she said.
FLVC, based in Mold, is a support organisation for 1,000 voluntary and community groups in the county.
It is calling on other businesses to ensure phone lines are protected after the hackers, believed to be based overseas, struck earlier this month.