asterisk users - Jan 2011 - Variable HANGUPCAUSE always empty with DAHDI

Hi,

I am using
Asterisk: 1.6.1.20
LibPRI: 1.4.11.4
DAHDI: 2.3.0.1 Echo Canceller: MG2
Wanpipe-Driver: 3.5.15
Sangoma-Firmware: 43 (A104d)

I handle some calls with my own PHP-AGI-Script. After a dial-command I 
use "GET FULL VARIABLE ${answeredtime}" or "GET FULL VARIABLE 
${dialstatus}" and get valid information. Sometimes "dialstatus"
has the
value "CONGESTION" OR "CANCEL". In this cases I tried to get
the
hangupcase with "GET FULL VARIABLE ${hangupcause}" but I always get an
empty result.

Examples:
[...]
[Jan 25 09:23:18] VERBOSE[28228] res_agi.c: <DAHDI/64-1>AGI Rx <<
GET
FULL VARIABLE ${answeredtime}
[Jan 25 09:23:18] VERBOSE[28228] res_agi.c: <DAHDI/64-1>AGI Tx >>
200
result=1 ()
[...]
[Jan 25 09:51:00] VERBOSE[1349] res_agi.c: <DAHDI/33-1>AGI Rx << GET
FULL VARIABLE ${dialstatus}
[Jan 25 09:51:00] VERBOSE[1349] res_agi.c: <DAHDI/33-1>AGI Tx >> 200
result=1 (CANCEL)
[...]
[Jan 25 09:51:00] VERBOSE[1349] res_agi.c: <DAHDI/33-1>AGI Rx << GET
FULL VARIABLE ${hangupcause}
[Jan 25 09:51:00] VERBOSE[1349] res_agi.c: <DAHDI/33-1>AGI Tx >> 200
result=1 ()
[...]

I expect the ISDN-Cause-Code here:
http://www.voip-info.org/wiki/view/Asterisk+variable+hangupcause

Does anyone have experience with that? Any idea why I do not get a valid 
hangupcause?

Thanks so far,
-Thorsten-