I just installed version 1.2 and it works.
There's one problem... it's not actually a widget. This plugin add the javascript code to the WP output. Unfortunately in my case it did that 5 times. I guess the plugin injects the code by using a hook and that hook is used multiple times. Hence multiple injections of the javascript code.
What I did was take the javascript code from their website (or confirmation email) and put it in a standard Text/HTML widget. And voilĂ , the Call Us button appears. User clicks the button, makes the choice for connection and the phone rings. I received the call with Skype on my Symbian mobile :)
I also tried the widget that could be downloaded from the invox site, but this does not work on WP 3.2.1 (it generates a 404 error).
Last point, it's very abstract with whom we're dealing here. Is it PBXplus or Invox or are they the same? Either way, thank you for providing this service! :)