asterisk users - May 2006 - Grandstream GXP-2000 call end

Hi

When I make a call with the Grandstream GXP-2000 through Asterisk (and SER) to 
landline using VSP, after I hang up the call the other party are still 
connected for another 30-40 seconds. I've notice that the SIP BYE is sent to
Asterisk, but Asterisk sends no SIP BYE on to VSP. When I use the SPA-941 the 
call terminates on the other right away soon as I hang up.
 
I have updated the GXP-2000 to latest firmware:
Software Version:  	  Program-- 1.1.0.11    Bootloader-- 1.1.0.1 

I'm wondering if this GXP-2000 problem or something else?

Thanks
-- Shaun