asterisk users - Jun 2007 - Calls audio stops with latest Gigaset C450IP firmware + voicemail

Hi,
I'm using Asterisk 1.2.18 on a Debian Etch box. I've noticed a very 
strange fact which causes a bad prob. When I get an inbound call, I make 
4 phones ring at the same time, one is a Snom while others are Gigaset 
C450IP with _latest firmware_.
When I get a call and answer with the Gigaset, a second call going to 
voicemail makes the first call received on the gigaset C450IP stop audio 
(it seems the call does not drop).
This does not happen if I answer the first call with the snom or with 
Siemens using older firmware or when gigaset are called in cascade 
sequence (Dial(SIP/1) then another Dial(SIP/2), etc...)
Is there anybody who can avoid this strange behaviour (I need to make 
all phones ring together)?

TIA

Giorgio