A method of operating a first terminal participating in a Mission Critical Push To Talk (MCPTT) group call in an off-network includes: receiving a first floor grant message configured to indicate a floor transfer from a second terminal to a third terminal; storing a Synchronization Source (SSRC) of the third terminal included in the first floor grant message as an SSRC of a next floor arbitrator; receiving a media packet or a floor control message; and, if a transmitter of the media packet or the floor control message is the next floor arbitrator, updating an SSRC of a current floor arbitrator as the SSRC of the next floor arbitrator.