A media proxy receive a first message from a near end of a path of a communications session, and before receiving a corresponding message from a far end, the media proxy is arranged to detect a blocking situation where another device in the path is awaiting the first message before forwarding the corresponding message. Detecting such a blocking situation enables it to be overcome, and enables the communication session to proceed. The media proxy can send a probe message to discover if there is another media proxy along the path causing the blocking. This is useful where the only information about the far end is the media path which is in the call set up, e.g. IP address and port. Sending the probe message can be under the control of a call server.