In the early stages, it will be a device enabling a single number that will operate in both environments, said Peter Kelly, Nortel’s president of enterprise networks for EMEA. In other words, it will be an either/or device, making calls from the enterprise WLAN when on campus (or in a WiFi hotspot) and on the cellular network when not.

That said, Brampton, Ontario-based Nortel clearly has the technology for roaming between the two environments, i.e. for calls to being in one and continue seamlessly across into the other. It already enables fast roaming between WLAN subnets and access points within its IPsec VPN offering, and adding in the cellular dimension would appear to be a distinct possibility.

We can already do real-time handover between 3G and WLAN from a radio perspective, but there are commercial issues of billing and call management that the mobile operators are gong to have to work out, said Kelly.

He refers to the issue of whether a cellular will be able to derive revenue from a call that, say, began on its network, then disappeared onto a corporate WLAN when the subscriber arrived at the office.

Then there is the reverse scenario, where a call will suddenly appear on an operator’s 3G network when a subscriber moves outside his or her company’s WLAN. The situation becomes even more challenging if public WiFi hotspots at places like airports are factored in.

Another interesting question, and one on which Kelly would not be drawn, is whether Nortel’s SIP client, which is available for RIM devices and shortly also for Windows Mobile 5.0 PDA/phones, will eventually find its way onto dual-mode handsets from the LG-Nortel joint venture.

It would certainly make sense for Nortel’s enterprise customers seeking cost savings on mobile voice traffic. Again, though, the company is probably exercising caution while mobile operators work out how they are going to bolster data and content revenues to compensate for the fall in voice income.

The SIP client is part of Nortel’s Multimedia Communication Server (MCS) collaboration suite, and as such offers much beyond simple VoIP calling. Kelly insisted, however, that all I’m saying is we have the MCS SIP client on BlackBerry devices today and going onto other PDAs, and that through LG-Nortel we have access to dual-mode phones. I’m making no statement on our strategy for dual-mode.