Disclosed is a method and system for establishing a session for a Push to Talk (PTT) over Cellular call (PoC). In the method, when an answer (response) mode of a PoC server and a PoC client are not in accordance with each other, an error message is transmitted to the receiving PoC server Moreover, call processing for a PoC call is improved by inserting a new data field indicating an automatic answer of a server into a payload of a Real-time Transport Protocol (RTCP) message, and a function of the terminal is extended using an added data field when the answer modes of the corresponding PoC server and PoC client are not in accordance with each other.