This invention provides a communication terminal device, a communication terminal receiving method, a communication system with the terminal device and a gateway which stabilize the time delay during VoIP communication and improve the voice quality. The invention comprises: a data packet unpacking unit; a receiving buffer; a decoding unit; a playing unit; and a central control unit. The central control unit comprises: a network state decision section for deciding whether the network is in a spike state according to the received data packets; a buffer adjusting section which predicts the subsequent data packets in one group of data packets when the network is decided to be in a normal state to adjusts the size of the buffer and predicts the subsequent data packets in one data packet when the network is decided to be in a spike state to enlarges the buffer.