Multiple Inbound Calling Problem

We running a calling card system, we are encountering a problem that l have been trying to solve without much success. Our system consist of Asterisk server with Digium TE210P interface. The system work find if only two concurrent user are on it, although it should be able to accommodate up to 5O simultaneous user but when the third user dial in the system send “Channel Unavailable” or “Circult Busy” message, what could be the problem here. Any help will be greatly appreciated.

zaptel.conf

span=1,1,0,ccs,hdb3,crc4
bchan=1-15
dchan=16
bchan=17-31
span=2,0,0,ccs,hdb3,crc4
bchan=32-46
dchan=47
bchan=48-62

loadzone=nl
defaultzone=nl

zapata.conf
channels]
language=de
switchtype=euroisdn
usecallerid=yes
hidecallierid=no
callwaiting=no
echocancel=yes
echotraining=yes
jitterbuffers=2
overlapdial=no
immediate=no
amaflags=default
busydetect=no
;busycount=5
callprogress=yes
useincomingcalleridonzaptransfer=yes
;busypattern=500,500
signalling=pri_cpe
callerid=“TELES” <44778522396>

internationalprefix = +
nationalprefix = +44
localprefix = +4477
privateprefix = +44778522396
unknownprefix =
;resetinterval = 864000
callgroup=1
pickupgroup=1

context=incoming
group=1
channel => 1-15
channel => 17-31

group=2
channel => 32-46
channel => 48-62

extensions.conf
[incoming]
exten => _ZXXXXX0,1,Answer()
;exten => _ZXXXXX0,n,Wait(1.5)
;exten => _ZXXXXX0,n,Playback(cc_technical_error)
exten => _ZXXXXX0,n,Set(CALLERID(number)=493062206099)
exten => _ZXXXXX0,n,DeadAGI(tele4me_cc.agi,2)
exten => _ZXXXXX0,n,NoOp(${cc_accountcode})
exten => _ZXXXXX0,n,Hangup()
exten => _ZXXXXX1,1,Answer()
;exten => _ZXXXXX1,n,Wait(1.5)
;exten => _ZXXXXX1,n,Playback(cc_technical_error)
exten => _ZXXXXX1,n,Set(CALLERID(number)=493062206099)
exten => _ZXXXXX1,n,DeadAGI(tele4me_cc.agi,3)
exten => _ZXXXXX1,n,NoOp(${cc_accountcode})
exten => _ZXXXXX1,n,Hangup()

log file ?