A substantial increase in your telephone bill is an indication your company couldbe the victim of Private Automatic Branch Exchange (PABX) fraud. Detailed billing will assist in identifying any potential unauthorised calls, usually International calls but they can also be National telephone calls. Another indicator is where customers trying to dial, in or employees trying to dial out, find that the lines are always busy.A PABX is a computerised system that manages an internal telephone extensions network.It is a highly flexible system as it can provide access to telephone services by dialing into the system from outside the PABX network. This service is called DISA (Direct Inwards System Access) and if enabled it permits employees to route national and international calls through the PABX with the cost of these calls being billed to the owner of the PABX. Access to this service requires the use of a PIN. This can however be abused and may result in unauthorised calls costing thousands of Euro. Most PABXs have engineering and maintenance access codes. If this access code is compromised the attacker will have total control of the system.If your PABX has voicemail and is DISA-enabled then it is susceptible to this form of fraud. To protect your PABX from this type of fraud, if DISA is not required ensure that it is disabled. If DISA is required, contact your PABX supplier or maintenance company, who can help you in configuring DISA properly.Toll Fraud Audit - this service is provided by your PABX supplier or maintenance company.Enable automatic logging of calls if available. This may help in identifying the extension number that is being used to compromise the PABX and it may also identify the source of the external call.Regularly check the log records for repeated short duration calls to the same number. This could be an indication of an attempt to attack your system.Activate PINs for voicemail, DISA and engineering access (if enabled) and change regularly.If possible, remote engineering access should only be permitted on a ‘call back’ basis. This will prevent unauthorised access to this privileged account.PABX Fraud is a criminal offence and in addition to reporting the incident to An Garda Síochána we would recommend that you contact your telecoms provider and your PABX supplier who will assist.