asterisk users - Jun 2007 - hangup during voicemail announcement drops all calls

Hi,
I'm using Asterisk 1.2.18 on a Debian Etch box.
I have two phones a Gigaset C450IP and a Snom 360. Suppose someone is 
calling the Gigaset phone and a second call comes and is redirected to 
the voicemail: if the new caller hangs up during voicemail announcement, 
Asterisk drops the first call.
This does not happen if the first called party answers the incoming call 
using the SNOM phone.

TIA

Giorgio Incantalupo