There are several possible solutions. First of all you can try to build you own SIP solution from free components such as ser, asterisk, vovida, … Will take you some time, and requires skilled professionals with SIP experience – otherwise you will just waste time&money.
Or you can get a complete commercial SIP solution (perhaps integrated with the billing). You can check Cisco SIP server – it is about 40K and is not billing integrated, or PortaSIP from Porta Software (www.portaone.com) – definitely cheaper and can be used standalone or together with PortaBilling.