Problem with Receiving Incoming calls from PSTN to Asterisk

[color=blue]Hi Friends,

I am Chandra from India. Thank you for your cooperation and for clear my doubts.

Now, I have installed Digium TDM04B card in my Asterisk server and configured. I have one landline number from PSTN. Now, I have connected that PSTN cable to my TDM04B first port. When I am making calls from outside to my PSTN number, sometimes Asterisk receving that call and sometimes, its not receiving. Why? Here I am giving my configuration of my files.

ZAPTEL.CONF contents:

loadzone = us
defaultzone=us
fxsks=1,2,3,4

ZAPATA.CONF contents:

[channels]
context=tutorial
signalling=fxs_ks
busydetect=1
busycount=7
relaxdtmf=yes
callwaiting=yes
callwaitingcallerid=yes
threewaycalling=yes
cancallforward=yes
echocancelwhenbridged=yes
rxgain=0.0
txgain=0.0
callerid=asreceived
language=en
usecallerid=yes
echocancel=yes
transfer=yes
immediate=no
group=1
channel => 1

SIP.CONF contents:

[300]
type=friend
username=300
secret=server
callerid="Server"
host=dynamic
context=tutorial

[general]
port=5060
bindaddr=0.0.0.0
context=default
disallow=all
allow=gsm
allow=ulaw
allow=alaw

EXTENSIONS.CONF contents:

TRUNK=Zap/g1
TRUNK=Zap/g2

[tutorial]
exten => s,1,Dial(SIP/350,30)
exten => s,n,Voicemail(350)
exten => s,n,Hangup

exten => 300,1,Dial(SIP/300,15)
exten => 300,2,Voicemail(u300)
exten => 300,3,Voicemail(b300)
exten => 300,4,Hangup

What is the solution? Please tell me. Looking forward to your response.

Thank you.

Regards,
Chandra.[/color]

does the console show anything when you call the channel ?