What is UC?
The communications divide

The communications world is split in two—between the things you do on the telephone and the things you do on the computer.

The split exists because most real-time (synchronous) communications--like telephone calls and voice mail--depend on one network, while message based (asynchronous) communications--like e-mail--depend on a separate, incompatible network.

Real-time (synchronous) communications run on the Public Switched Telephone Network (PSTN).
Asynchronous communications run on packet-based networks like Ethernet (IP).

The split creates problems, lots of them. Phones aren't as intuitive as they should be: just try to start a three-way call without hanging up on someone. Computers can check your e-mail, but not your voice mail. And then there's the enormous cost of purchasing, maintaining, and upgrading two complex infrastructures.

To get your phones and your computers talking, you'd have to tear out your entire telephone system, dump your PBX, replace every desk phone, swap out every phone jack. In short, you'd have to start from scratch.