A very neat idea but, way over engineered.
- if you pick up the phone at work, you can be marked unavailable on others' phones / local IM / ..., not so much on the mobile
- you have to pay for the connection you're making to your mobile (so basically paying for answering the call) - if you're somewhere where you can pick up a voip phone, that's not the case anymore
- during lunchtime, you could redirect people to an IVR "I'm at lunch - if it's urgent press "1", otherwise call be back in 15"
- you might not want others to call your mobile number directly (or you have a phone presence in another country, so the call goes through your pbx anyways)
- you can limit the noise in the office automatically if your phone doesn't ring when you're not there
- people calling you don't have to wait X minutes to find out you're really not there, and not running to the phone
There are many cool features, most of which you don't need really. But if your work relied phone communication 99% of the time, you could probably come up with 50 more points about what to do with this system.
I think that's just neato!
I started out working on a similar feature for my own VBX platform (OtherNum.com) but once Jeff showed me OpenVBX I immediately ported all my "plugins" over including my integration with Chirbit.com and MyCaption.com.
I wrote more about the impact OpenVBX had on my startup OtherNum on my tumblr: http://tumblr.gleep.org/
Ultimately, the 4square piece is a tiny piece of the OpenVBX world but I think what it shows is the potential to integrate your phone infrastructure with the world around it.
