PABX hacking/phreaking/toll fraud is becoming increasingly widespread and is costing businesses globally £40 billion a year in unexpected telephone bills, £1.2 billion of which is charged to UK companies.
PABX dial through fraud is carried out by fraudsters who hack into a phone system, often at night or at weekends when it is least likely to be detected, running up huge costs for the unsuspecting owners. There are even websites dedicated for phone system hacking/PABX dial through fraud which is now becoming big business for organised criminals rather than just a wheeze for internet geeks.
These fraudsters know that most companies are oblivious to the threat of phone system hacking. Telephone network operators whilst perhaps having fraud prevention departments to warn users about unexpected and excessive usage are generally unsympathetic to companies that have been hacked and make the unfortunate victim pay.
You should ensure you take care of telephone system security to avoid such costs for your business.