Why use PBX? Go for VoIP instead. It's cheaper, more portable, more flexible, etc. You have a much wider range of hardware choices, and you can even use soft phones on your computer and mobile devices (Your iphone or even iPad can even become an extension). Route calls to your home or Skype, record calls, have voicemail messages emailed to you, and much more.
You can even take a hardware phone with you as you travel and have instant access to your calls from any Internet connection. Some phones (such as the Cisco 525G have built-in Bluetooth and Wifi) letting you make and take office calls, wirelessly, though its more elegant to use voip or route to your mobile device. But still. I did this for the Qwebec Expo last year and it worked perfectly.
Try
http://www.ringcentral.com
or
http://8x8.com