asterisk users - Oct 2011 - Still having trouble to configure gxw4108 with asterisk 1.8 need enlightenment

Dear all,
I'm still having trouble using asterisk with the grandstream gxw4108,
in the gxw4108 I'm using 1 stage dialing in the profile1 I already
type my asterisk server address 192.168.14.80 and my grandstream IP is
192.168.101.184

here's my asterisk config files

SIP.CONF
[1401]
type = friend
username = 1401
secret = 1401
host = dynamic
context = kantor-mtx
insecure = port
nat = yes
dtmfmode = rfc2833
canreinvite = yes
notifyringing = yes

[1402]
type = friend
username = 1402
secret = 1402
host = dynamic
context = kantor-mtx
insecure = port
nat = yes
dtmfmode = rfc2833
canreinvite = yes
notifyringing = yes

EXTENTION.CONF
[kantor-mtx]
exten => 1401,1,Dial(SIP/1401,60)
exten => 1402,1,Dial(SIP/1402,60)
exten => _NXXXNXXX,1,Dial(SIP/${EXTEN}@1401)
exten => _0813XXXXXXXX,1,Dial(SIP/${EXTEN}@1402)
exten => _0812XXXXXXXX,1,Dial(SIP/${EXTEN}@1402)

with this configuration files the result :
1.Sometime when i'm dialing to the PSTN line, not the PSTN ringing,
instead the extention is ringing
2.when I restart both asterisk and gxw4108, it's succes when dialing
to PSTN, but when I try to dial the extention it's seems dialing to
PSTN

(I also configuring to separate my incoming and outgoing call but
still the same error occurs)

my question is :
1.is it my gateway is broken or malfunction ?
2.When testing this configuration I'm using only 2 PSTN line instead
of 8 lines provide by the gateway, is this can cause problem in my
configuration ?
3.Or there is some other configuration for my asterisk and gxw4108,
instead the one I'm using would kindly to share in here

Thank you very much for your guidance and sorry if my English is bad
-- 
Regards
Samuel Sappa,