A Voice over Internet Protocol (VoIP) system having a dynamic gain control function and a method for providing a gain dynamically using the same system wherein, in the process that Pulse Code Modulation (PCM) data are transformed into a VoIP packet or vice versa, a gain value is dynamically assigned according to the type of terminal equipment utilized and the PCM voice data can be amplified and outputted according to the assigned gain value. The system includes a private exchange having a gain table and a VoIP gateway for the receiving the gain value from the private exchange in order to amplify the PCM voice and transform the amplified PCM voice data into the VoIP packet or convert VoIP packets to PCM voice data and amplifying the PCM voice data.