asterisk users - Feb 2007 - unwanted chanspy: strange behaviour

Hi,
I have an Asterisk box with a Sangoma PRI on a Debian distro. It may 
happen that user A tries to call B but for some reason the call drops 
and A is connected to another call between C and D. User A can only hear 
what C and D are saying, like a sort of "unwanted  chanspy".
Is there anybody who experienced a strange behaviour like this?

TIA

Giorgio Incantalupo