The idea is that if you have a business PBX (trixbox), the trixbox system will automatically "sense" when you are in the office and send calls to your usual extension. However, when you leave the office (with your trusty iPhone of course), the system detects (via Blutooth) that you aren't in the office and then trixbox automatically forwards your business calls to your iPhone. Sweet!
Vittles explains it best:
If impressing your friends happens to be your thing, then you'll enjoy watching them all wet their pants today after you show them this little trick. It's been almost two years since we first introduced Bluetooth Proximity Detection to the Asterisk community. Because we can't seem to get enough programming torture lately, we decided to revisit Follow Me Phoning by taking it to a whole new level with an all-new set of devices. If you're new to all of this, what we're talking about is the ability to stroll out of your home or office and have your telephone calls follow you on your cellphone automatically... without touching anything. And, when you return, the home or office phones start ringing again just as if you never left. Won't your boss or spouse be thrilled? What makes all of this possible, of course, is Bluetooth which happens to be running on your TrixBox/Asterisk server and on your brand new iPhone.
Now head on over to Vittles' site for the full-on recipe!
Oh and speaking of iPhone recipes - ever wonder what happens when you take an Apple iPhone and stick it in a kitchen blender? Well, wonder no more! Check out this video (yes it's real):