Call interrupted by incoming call

I have a customer that is reporting that one handset is interrupting an ongoing call with a incoming call. They say they are getting disconnected from the person they are talking to by another person who just called in. The asterisk box is updated with all the latest updates running asterisk 11. I have found nothing to explain it and the other stations are not having this problem. We have replaced the phone with a new unit. Also we have switched them out with a different model handset. There is nothing about their extension that is different in setup than the other handsets. I have searched the forums for similar problems but haven’t found anything. Any help or ideas would be appreciated.

Is there a concurrent call limit?

There has to be a difference between this extension and the rest if this one if the only one doing it. I would have been surprised it changing the handset fixed the problem, since the disconnect sounds like it’s happening at the PBX.

Can you get a /var/log/asterisk/full log extract that shows what is actually happening when the call drops? If so, look through that and find the context and action that is hanging up the phone.

Are these dahdi trunks?

They are sip trunks.

I have looked though the logs and am by no means a asterisk expert. I usually don’t have any trouble finding the answers to my problems by searching the forums. I have looked at the number they gave me but do not see any difference between the extension that is having trouble and any of the other extensions that have called that number today. I have asked them to get me the incoming number that interrupts their call.