This paper describers the results and lessons from the voice over IP trial service on the Korea Telecom's VoIP Testbed. The testbed was made up of four different vendors' systems and solutions constituted four separate zones. Even though the backbone network of the testbed was not commercial IP network, we could comprehend some engineering parameters essential to packetized voice QoS. And we got some know-how. These kinds of results will be much help to traditional telco confronted with many difficult issues especially on packet voice network.