Yes Kputz show you the good way A2billing have all these features I think
but I don't use it because it's not simple and I realy don't understain half of features
(this does mean interface and asterisk is on same server)
(yes sure a billing switch is made for billing)
Calling Card System
(this does mean the switch work with prepaid account and cut when there is no more credit)
I have make my own switch for my use here voipcom.fr
this is the frontend for client login they can see sip informations, balance
and print detailed month invoice that's what I need to sale my communications !
backend is already very simple just five minuts to create new customer
or changing some rates and routes.
I have make this appliance with php/mysql and agi this is not very difficult
but it take many time.
I have not include callback and DID features because I don't need but it's not so hard !
I think you will save money using A2billing and learn how to use it
better than pay 2 monthes dev.
but if you realy whant your own system contact me.