A method of Mission Critical Push to Talk (MCPTT) service performed by a mobile terminal includes: generating, at the mobile terminal, a group call announcement for off-network Mission Critical Push to Talk (MCPTT) service, the group call announcement for off-network MCPTT service including information of a period of the group call announcement for off-network MCPTT service; controlling a transceiver to periodically transmit, to one or more mobile terminals, the group call announcement for off-network MCPTT service based on the period; in response to the transmitted group call announcement for off-network MCPTT service, receiving a response from the one or more mobile terminals; and establishing, based on the response from the one or more mobile terminals, a group call with the one or more mobile terminals by setting a direct wireless communication with the one or more mobile terminals.