Your company's telephone system could be providing a 'back door' for hackers to access internal networks, according to a telecoms expert.

And the cost of phone hacking in the UK has now overtaken that of credit card fraud, estimated at £1.5bn, said Russell Marriott, the managing director of telecoms firm Eastern Voice and Data.

He said legacy telecoms systems could act as a 'forgotten back door into your local area network', with hackers able to get in through poorly-protected voicemail boxes or insecure data ports on networks to which the systems are connected.

'These hackers are employing technology on an industrial scale, dealing with robots, bombarding sites and looking for weak ways in,' said Mr Marriott, whose firm has clients across East Anglia, from Norwich Theatre Royal to King Edward VI Grammar School in Chelmsford.

'It is common for people to have no maintenance support [for telecoms systems], but they need specialist advice.'

Mr Marriott said cloud-based phone systems, which can encrypt and store call recordings, are the future even in areas of the region with current poor connectivity. 'If you are an SME you are going to be getting the same support as the big players,' he said.

He added that the planned phase-out of all old ISDN telephone systems by 2025, in favour of new SIP systems which work through IP addresses, was an opportunity to 'get rid of the weak spots in our offices'.

The inaugural meeting of the Norfolk Cyber Security Cluster on Norwich Research Park also heard from Chris Brown, the solicitor who defended the teenaged Talk Talk hacker, who said companies which suffer cyber security breaches risk being both criminalised and victimised by the incident.

Mr Brown, chief executive of Fosters Solicitors, said the crime was the first he had encountered with this ability.

Meanwhile Geoff Connell, head of information management and technology at Norfolk County Council and president of public sector IT society Socitm, said 'doing the basics properly' and sharing best practice were key to protecting companies from security breaches.

Cyber security will be the subject of a breakfast networking event being hosted by Innovation Martlesham, the high-tech cluster for firms based on and around BT's Adastral Park research centre near Ipswich, in association with TechEast.

Guest speaker at the event on Wednesday, July 5 will be Paul Maskall, cyber security adviser at the Cyber and Serious Crime Directorate for the Norfolk and Suffolk constabularies.