Recent research suggests that 44 per cent of the world's corporate telephone lines may be IP-based by 2008, prompted by cheaper equipment and better integration with existing TDM-based telephony systems.

But a separate survey of Nortel customers found that 48 per cent of firms remain unconvinced of VoIP's ability to deliver an adequate return on investment.

Most VoIP installations are deployed at new sites or offices with no existing telephone system or network infrastructure so, in these cases, building one converged voice and data network instead of two separate systems can deliver significant cost savings and management efficiencies.

But most IT managers have been reluctant to write off the substantial investments already made in analogue PBX systems by replacing them with VoIP.

Another barrier to adoption has been the high price and limited availability of IP handsets, forcing firms to seek alternatives in the form of software-based telephony apps and ways to attach legacy analogue handsets to IP networks.

Follow all the arguments for and against VoIP in our Special Report.