Toll free routing in the US is handled by Inter-exchange carriers such as Sprint and AT&T. A company which wants to provide toll free number service chooses a Responsible Organization (RESPORG) to maintain them. A RESPORG is actually an Inter-exchange carrier that is going to provide the necessary toll free routing services.
When a customer dials a toll free number, each of the digits is analyzed and processed by the Local Exchange Carrier (LEC). The fact that the number is a toll free call is recognized by the service switching point (SSP). A SSP sends call information to the Signal Transfer Point (STP), which in turn enquires the Service Control Point (SCP) about the destination of the call.
A company can share its toll free numbers among various Interexchange carriers. The task of determining which Interexchange carrier handles the call is performed by the LEC. Once the carrier is traced out, the call is forwarded to the carrier’s point of presence (POP). The routing of the call is then performed by the carrier’s SCP using a protocol called Signalling System 7. The call is finally routed to the company’s trunk lines, wherein a PBX system handles the call.