Software Call Center

I want to set up a software call center with the following features:
Customers buy call time (prepaid)
Any customer with a valid account (through caller id), and a credit balance would be given a dial tone.
He can them proceed to dial a number he wants to call, the software should then automatically make the call, and establish a connection (through a VOIP phone connected to my network).
I would also require standard call center management features like account management, call times, account balances etc.
Can anybody suggest an appropriate software for me to buy?