A method, user equipment, and system are provided which can open an ad-hoc Push-To-Talk (PTT) over Cellular (PoC) session for a call target group whose group identifier is managed in a remote network domain when the ad-hoc PoC session is opened. A PoC client requesting session initiation sends, to a first PoC server of a home network, a session INVITE message containing information of at least one group identifier. The first PoC server checks the group identifier information and sends a group list information request message to a second PoC server hosting a group identifier when identifier information of a network group other than the home network is included. The second PoC server acquires list information about a group in the request message and sends the information to the first PoC server.