When I worked in VoIP dpmnt. of some IM provider users were reporting issues like this. Sometimes caller and callee were connected to other party in a random manner. It turned out this strange behavior was caused by bug in PBX software- TDM channels were linked incorrectly under heavy load on E1 links. AFAIK this bug is known for years and it is still not fixed. But... I guess it's not related to your case, since vendor of this PBX is Polish.