Asterisk dropping call- Span 1 got hangup request, cause 16

Dear All,

My Asterisk system intermittently give this message “Channel 0/X, Span 1 got hangup request, cause 16”. Where X is channel number. And after then it drops the customer call. It has happen 4 times today.

I am running FC10, Asterisk 1.4.22.2, Dahdi-linux 2.1.0.4 and Dahdi-tools - 2.1.0.2 and I have my

extension.conf as:

exten => _3434343X,1,Answer()
exten => _3434343X,n,AGI(127.0.0.1:9001)
exten => _3434343X,n,Hangup()

chan_dahdi.conf:
[channels]
context=default
usecallerid=yes
hidecallerid=no
callwaiting=yes
usecallingpres=yes
callwaitingcallerid=yes
threewaycalling=yes
transfer=yes
canpark=yes
cancallforward=yes
callreturn=yes
echocancle=yes
echocancelwhenbridged=yes
relaxdtmf=yes
rxgain=0.0
txgain=0.0
group=1
callgroup=1
pickgroup=1
immediate=no
switchtype=euroisdn
context=from-pstn
group=0
echocancel=no
signalling=pri_cpe
channel =>1-15,17-31

dahdi/system.conf:
loadzone=us #I think I should change this to IN
defaultzone=us # should change to IN; Does this really matters?? Pls suggest
span=1,0,0,ccs,hdb3
bchan=1-15,17-31
hardhdlc=16

I have one full PRI line connected with the system. I am in India and have Tata Teleservices line.

Can somebody please enlighten me about this issue and why is this happening. How can I stop such thing happening so that I can take my small application (simple IVR based) to many users.

Thanks,

Best Regards,
VKM