A Push-to-talk over Cellular (PoC) communication session is established between an inviting PoC client and at least one invited PoC client using an IP Multimedia Subsystem network. Establishment involves using a pre-established session procedure to negotiate media parameters between the inviting PoC client and a participating PoC server for the inviting PoC client. At the inviting PoC client, a Talk Burst Control Protocol (TBCP) or Media Burst Control Protocol (MBCP) message is sent directly to its participating PoC server, the TBCP or MBCP message containing the PoC address of the at least one invited PoC client. At the participating PoC Server, in response to receipt of the TBCP or MBCP message, a SIP INVITE message is generated and sent towards a participating PoC server for the at least one invited PoC client.