Dynamic Extension, also called cell phone twinning, is a standard feature in 5000/250 now. When a person's desk phone rings, the cell rings also. As TE said, instead of a desk phone it can also be tied to a PHANTOM EXTENSION (virtual device) or a HOT DESK (phantom that can log into any phone when he's at an internal desk). It's free, works great, and unanswered calls go back to his PBX voice mail, where pressing 0 actually can do something. Works okay on any line, but even better on SIP or PRI where, depending on carrier, it can even propagate the original caller ID out to his cell.
The way it's built you can't normally make outbound calls through the PBX but I have a work-around to provide that if you need it.