Are "zombie" calls causing this?

Recently, I upgraded our asterisk box from 1.4 to 1.6. Since the upgrade a few users are experiencing “ghost calls” where they pick up the phone and nobody is there. When I check the full log I see a lot of zombie calls for that particular extension. How can I track the origin of this issue?

Depends on the connection technology. For SIP, the history facility should show masquerades, I think.

It appears they are getting generated from outside, dahdi channels, to SIP channels. Would something else be causing this?

Are those users agents of a queue?

Nope… The only difference for the uses are their phone is set to ring with someone else. I have set the internal and incoming calling dial plans to how they used to be, but still getting these calls.