A routing service method in a public switched telephone network (PSTN) system uses a stand-alone gateway interworking with disparate central office lines. The stand-alone gateway checks whether a call service can be provided over the Internet with reference to an internal database having periodically updated call service state information in response to a call service request from a calling private branch exchange (PBX) connected to the stand-alone gateway via a central office line. If the call service cannot be provided over the Internet, the stand-alone gateway transmits a call service request, together with a public switched telephone network (PSTN) rerouting control request, to the calling PBX. The calling PBX transfers the call to a called party over the PSTN in response to the PSTN rerouting control request.