asterisk users - Jul 2008 - Asterisk hangup not working on inbound calls

Hi,
I have an Asterisk 1.2.18 box with a TDM400P card.
If I make a call and then I hangup the phone, the call ends correctly 
but if I receive a call and I hangup the phone the other party does not 
get the hangup signal from Asterisk.

Is there anybody who can explain this strange behaviour?

Thank you.

Giorgio Incantalupo.