Crossed Lines with SIP channels

From my understanding of SIP, it nearly impossible for regular SIP channels to get “crossed”. Remember the old days when you sometimes get a “crossed line”, and you could hear the other guys having a conversation. In our 100% SIP environment, one customer is experiencing that quite often. I have listened to the call recording and fair enough, its full on crossed… lots of “hello? hello? oh sorry, who is this? im sorry…” It seems like the following happens:

  • ExtA calls ExtB
  • ExtB answers, “hello…?” but is connected with CallerX
  • ExtB cannot hear ExtA, but ExtA can hear ExtB, and cannot hear CallerX
  • then, ExtB puts the phone down, and this connects ExtA with CallerX

That was it, they put the phones down after that. We dont have any more information on this callerX expect that ExtB said “I think you have the wrong number” to CallerX and CallerX said “i think you have the wrong number” to ExtA

Any clues? has this happened to any of you? Seems impossible right!