Presence technology can bring a lot of value to companies, but here's a weakness: You still can't tie mobile workers into a presence system with complete confidence that you're always seeing them. Companies can extend a call to an employee's mobile phone, and if an incoming call is routed into the call server and then out to the cell phone, when that person answers it will switch to "on-the-phone" status in their colleagues' unified communications portals.

But what if that mobile worker calls someone outside the company on a mobile phone? The system can't tell that the person is on the phone, and his presence status doesn't change. The only way around that problem today, says Jeri Korkki of IBM Global Technology, is if mobile operators would convey on-hook/off-hook status to enterprise systems. Yeah, good luck working that out with the carriers.

Another alternative, if the person's mobile device has a UC client on it, would be to set up the call server so it puts one leg of a call out to the user's cell phone and the other leg to the person the user is trying to reach. Then the call server would know the mobile user's status and could reflect it.

If presence is the dial tone of the future, what if dial tone and busy signals couldn't be relied on to reflect the true status of a person on a call? What if you called someone and got a busy signal even though he was waiting for your call? As presence becomes widespread, having accurate mobile presence will matter in the same way.