Sometimes, customers can not call in?

Hello, Guys:
I have lost many customers because of my asterisk box. sometimes, customers call in , operators can answer the calls. but sometimes, customers can not call in. it seems that nobody picks up the phones. actually, the operators do not know that the calls is coming in. I think, maybe the zapata config files have a wrong setting. the channles sometimes are not available. anyone knows how to solve problem? please give me a hand.
Regards!
zhu

well, give us a hand by perhaps posting your config files and/or debug logs…we cannot read your mind.

read this throughly: forums.digium.com/viewtopic.php?t=4208

Hello, Guys:
I posted my part of dial plan and zatapa.conf files.
[ext-group]
include => ext-group-custom
exten => 1,1,Macro(rg-group,ringall,60,304-305-306-307-303)
exten => 1,2,Goto(aa_1,s,1) ; jump

[ext-local]
include => ext-local-custom
exten => 301,1,Macro(exten-vm,301,301)
exten => ${VM_PREFIX}301,1,Macro(vm,301)
exten => 301,hint,SIP/301
exten => 302,1,Macro(exten-vm,302,302)
exten => ${VM_PREFIX}302,1,Macro(vm,302)
exten => 302,hint,SIP/302
exten => 303,1,Macro(exten-vm,303,303)
exten => ${VM_PREFIX}303,1,Macro(vm,303)
exten => 303,hint,SIP/303
exten => 304,1,Macro(exten-vm,304,304)
exten => ${VM_PREFIX}304,1,Macro(vm,304)
exten => 304,hint,SIP/304
exten => 305,1,Macro(exten-vm,305,305)
exten => ${VM_PREFIX}305,1,Macro(vm,305)
exten => 305,hint,SIP/305

zapata.conf :
[channels]
language=en

; XTDM31B Port #2,3& 4, plugged into PSTN and Port #1 to Extension
;
context=from-pstn
signalling=fxs_ks
faxdetect=incoming
usecallerid=yes
echocancel=yes
echocancelwhenbridged=no
echotraining=800
busydetect=yes
busycount=8
callprogress=no
group=0
channel=2-4

context=from-internal
signalling=fxo_ks
usecallerid=asreceived
echocancel=yes
echocancelwhenbridged=no
echotraining=800
callerid=Free Net Business Solutions S/B
callgroup=1
pickupgroup=1
group=1
channel=1
threewaycalling=yes
transfer=yes
relaxdtmf=yes
I think maybe hint problem. can i use dial() insteand of hint. please give me a hand.
regards!
zhu