Our communications systems operate on two totally separate levels- the things you can accomplish on the telephone and the things you accomplish on the computer.  The goal of unified communications is to unite these two worlds so that they complement each other and work together.
The gap in the systems exists because most real-time (synchronous) communications including voice mail and telephone calls, rely on a network.  Message based (asynchronous) communications, such as e-mail, depend on a separate network which is incompatible with that of the real-time communications network.
Synchronous communications operate on the Public Switched Telephone Network (PSTN).
Asynchronous communications operate on packet-based networks like Ethernet (IP).
This divide in communications systems creates a variety of problems.  It’s like having two people attempting to get one job accomplished but not allowing them to work together.  Phones aren’t as instinctive as they need to be while computers can check your e-mail, but not your voice mail.  These issues, along with the cost of purchasing, maintaining and keeping these separate infrastructures up-to-date, can be an enormous task and inefficient for your company.
How do you unite these communications systems?  One option would be to start from scratch- remove your entire telephone system, dump your PBX, switch every desk phone in the office, and replace every phone jack OR…