Trunk g0 can not work with more than 5 FXOs for outbound calls

hello,
our customers reported one problem. the problem is this,when put more than 5 FXOs with openvox A1200P, the 1,2,3,4 channel worked with g0, but rest of channels(5,6,7,8) can not work. the inbound calls works, but the outbound calls do not work. if change to this format in zap identifier:zap/5 or zap/6 instead of g0 or g1, then it works. this is my setting.
-----------zapata-auto.conf---------------------
to be #include-d by /etc/zapata.conf that will include the global settings
;

; Span 1: OPVXA1200/0 “OpenVox A1200P Board 1” (MASTER)
;;; line="1 OPVXA1200/0/0"
signalling=fxs_ks
callerid=asreceived
group=0
context=from-zaptel
channel => 1
context=default

;;; line="2 OPVXA1200/0/1"
signalling=fxs_ks
callerid=asreceived
group=0
context=from-zaptel
channel => 2
context=default

;;; line="3 OPVXA1200/0/2"
signalling=fxs_ks
callerid=asreceived
group=0
context=from-zaptel
channel => 3
context=default

;;; line="4 OPVXA1200/0/3"
signalling=fxs_ks
callerid=asreceived
group=0
context=from-zaptel
channel => 4
context=default

;;; line="5 OPVXA1200/0/4"
signalling=fxs_ks
callerid=asreceived
group=0
context=from-zaptel
channel => 5
context=default

;;; line="6 OPVXA1200/0/5"
signalling=fxs_ks
callerid=asreceived
group=0
context=from-zaptel
channel => 6
context=default

;;; line="7 OPVXA1200/0/6"
signalling=fxs_ks
callerid=asreceived
group=0
context=from-zaptel
channel => 7
context=default

;;; line="8 OPVXA1200/0/7"
signalling=fxs_ks
callerid=asreceived
group=0
context=from-zaptel
channel => 8
context=default


after making outgoing calls, the asterisk console shows the zap/g0/ answered calls, but the far side devices can not get calls.
--------------asterisk console=================
– Executing [s@macro-dialout-trunk:20] Dial(“SIP/1000-0a25d6b0”, “ZAP/g0/1314588XXX|300|”) in new stack
– Called g0/1314588XXX
– Zap/2-1 answered SIP/1000-0a25d6b0
trixbox1*CLI>

actually, the channels is connecting to channel 6.anybody knows why the channel can not jump to 6 because channel 6 is under group0?
thanks!
james.zhu