If user has Jabber client installed, this can be done by simply adding the phone number into the URL form like this:
<a href="TEL:1234">Call 1234</a>. When user click on the link, it will launch their Jabber client.
If you are trying to implement this with Jabber SDK for web, then Message Passing might be something you are looking for?
http://developer.chrome.com/extensions/messaging
Sorry I never play with extension, assuming you can listen to the events via Message Passing from either side and do the needful. Ex: When call state changes, devices registered, etc.
Regards,
Howard