Disclosed is a method for enabling a terminating-side Push-To-Talk (PTT) over Cellular (PoC) client to open a PoC session by using a pre-established session, and to maintain the pre-established session when the terminating-side PoC client terminates the open PoC session, and a PoC system and a PoC User Equipment (UE) for implementing the same. To this end, a REFER BYE message is used to terminate the PoC session while keeping the pre-established session maintained. The REFER BYE message must include information, such as pre-established session parameter information, a session IDentifier (ID), etc. Also, the REFER BYE message is transmitted from the PoC client to the PoC server, and the PoC server receives the REFER BYE message, checks the contents of a header in the received message, and terminates the PoC session while keeping the pre-established session maintained when a specified condition is satisfied.