This work gives an over view of IP telephony and explain the existing difficulties in implementing VoIP services & migrate the TDM base telephony to IP telephony. When the architecture is deployed in IP based network. The causes of these problems are explained with three approaches used by carrier to solve interoperability. They are TDM base telephony to IP telephony by using soft switch and voice gateway, using different audio codec's for better voice quality, band width used in IP telephony.