The Department for Work and Pension's (DWP) newly installed IP telephony platform is now handling more than one million calls a day.
The landmark figure was reached following the technology's installation at 435 sites around the UK serving over 35,000 staff.
The Integrated Communications Network Services (Icons) programme being implemented at the DWP is one of the largest converged data and voice networks in northern Europe.
The system, provided by BT using IP phones and network hardware from Nortel, is part of a two-year DWP IT transformation programme designed to extract the maximum value from IT.
The DWP's two existing networks are being replaced by a single Voice over IP network capable of carrying data, voice and video.
This will allow the department's frontline staff to more effectively manage their workflow, reduce data-access times and facilitate team working and collaboration via video-conferencing.
To date, over 24,000 new handsets and nearly 4,000 fax and other analogue devices are in use over the network providing more integrated communications across e-mail, voice, instant messaging and video.
Employees have also been given greater scope for flexible working with the network, with the potential to link call centres and enable home and mobile working.
BT has upgraded over 1,000 DWP sites to connect existing legacy services to the converged wide and local area network.