More than 100 sites are using an NHS broadband network, installed as part of the £12.7bn National Programme for IT, to make voice over IP (VoIP) phone calls.

The service became available on the N3 NHS broadband network in July 2007 and has seen a 600 per cent increase in call volumes over the past six months.

The service allows clinicians and administrators in NHS trusts and those GP surgeries that are connected to call each other for free between fixed lines. Trusts are not forced to use it but can opt in if they wish.

Len Chard, N3 Programme Manager at NHS Connecting for Health, said: "An increasing number of NHS organisations are taking advantage of our voice offerings and reducing the cost of their telephone calls."

Chard says the service also provides considerably cheaper mobile phone connections between sites.

Up to 70 per cent of an NHS Trust's call charges are for landline to mobile calls, which become signficantly less expensive under the system.

From December, organisations will be allowed to transfer to the system while keeping hardware from a legacy Cisco system.

More than three million calls have been made across the system already, a number Chard hopes will triple by April next year and increase to 100 million by May 2011.

N3 is the NHS broadband network installed by BT in 2004 as part of the National Programme for IT. It is one of the parts of the scheme that was completed on time and on budget.