No idea. But it seems more likely that it's a problem with notification on Jabber or the phone rather than phantom calls occurring. At any rate, you could certainly pull CDR records and see if the calls actually came in over your gateway or were sent from another phone. Then I would see what happens when you do test calls to these problem phones/Jabber clients. Do they get notified of your test calls ok (maybe only the phone does and not Jabber, or vice versa)? I know this isn't much help, but I would do this first to make sure you know exactly what the scope of the problem is- this should help you get closer to the source of the problem.
