Hi,
I have already configured two successful calling card systems using a2billing , I can configure a2billing as calling card services , call back services and whole sale termination .. moreover I can also perform ss7 configuration setup to terminate TDM calls on your asterisk box and perform codec as well coordination with voice carriers technical/NOC team to setup trunk .. I am purely from voice background , I am always available messengers like skype , msn , whatsapp and tango to provide 24/7 support to my clients