A transmission method of a femtocell includes the following steps. The femtocell receives and temporarily stores multiple real-time transport protocol (RTP) packets from a UE in a buffer. When the RTP packets are temporarily stored for a time period, the femtocell samples the buffer at a sampling rate to generate multiple CS data packets. The CS data packets include a current CS data packet. When there exists no previous CS data packet received from the UE, the femtocell calculates a timestamp of the current CS data packet according to a current timestamp. When there exists the previous CS data packet, the femtocell calculates the timestamp of the current CS data packet according to the timestamp and a connection frame number (CFN) of the previous CS data packet and a CFN of the current CS data packet.