Don't forget you need to have a solid vendors or about $500 millions to become a vendor your self ;-)
There is a some systems that "ready" to go but in many cases you will not be able to find some features that you want... + they can't hold a lot of call's.
If you need a carrier grade solution should look in to Alcatel, Sonus and GenBand products for switching equipment and as of Billing and Provisioning there is a lot to pick from but I can recommend that works... is AVA VoIP billing.
You can find they demo here http://avavoip.com/index.php/live-demo
good solution to start with.
P.S. Unforchantly Cisco does not have any good carrier grade products for VoIP but they good for SMB and Enterprises.