A2billing question

Hi
I have confiured A2billing , I can dial no. and it prompt me for the pin and it works fine,but when I try dialing any no. is usa followed by a # sign it says
" the no. you have dialed is currently unavailable …"

My A2billing TRUNK settings are like

TRUNKCODE=default
PREFIX=1NXXNXXXXX
PROVIDERTECH=IAX2
PROVIDERIP=exgn (where exgn is the trunk name defined in AAH amp portal)

Please let me know where im wrong, I have not configured any RATE CARD uptill now,but i guess it should be able to dial

Thanks
Hasnain