[on-asterisk] trixbox/a2billing sip/iax call

[M . Arshad khan]

Hello,
I have installed trixbox 2.6 and a2billing 1.7 and succesfully making calls 
from sip phone to pstn using sip trunk, but can not make sip to sip calls for 
free/charge, any guru can help here.
Arshad