Cell phones are naturally morphing into the de facto business phone for many workers.

Some enterprises consider this problematic on a couple of counts. There's the obvious cost associated with paying for the use of two separate phone networks and the handsets to match.

More important, because cell phones traditionally haven't been part of the enterprise communications network, many employees have formed their own little "cellular islands" of voice communications over which the corporate IT department (and the business in general) has little or no control.

Folks in sales and other customer-interfacing positions, for example, may store contact lists locally on their phones. Their customers may know only the employees' cell phone numbers, which are often the most efficient way to reach someone live.

So when these employees leave the company, say, to go work for a competitor, well, many of the business' customers go right along with them.

"That situation presents a very real business case [for enterprises to converge their phone networks]," says Abner Germanow, director of enterprise networks research at global researcher IDC.

He adds that "enterprises are looking at the spending that goes into voice and trying to make sure someone is at the other end to answer" to avoid the growing costs of phone tag.

Indeed, rather than trying to modify users' behavior, some enterprises have just come to the conclusion that extending the calling features, phone numbers, and dial plan of the corporate PBX is the way to go.

Some emerging solutions to creating a single business phone number that can reach you on your cell phone require a wireless data service to integrate IP PBX functionality and deliver voice calls over IP on the cell
network. Others use legacy dual-tone multifrequency (DTMF) tones as the lowest common denominator calling technology so that you can basically push calls to any phone.

Some solutions use a find me/follow me approach; Germanow says that enterprises seem to prefer multiple phones ringing simultaneously to avoid the delay associated with the system hunting around for the intended recipient.

"For most people, if they're going to end up in voicemail, they want to get there as fast as possible," he says.