VoIP service within the PBx is be used at the following parts of the Charles University:

In order to operate from PBx, the existing ISDN-PRI card is used, while trunk for transfer to VoIP ends in a gateway. A voice gateway enables a link between a telephone and packet network, meaning it carries out all the requisite physical translation and voice compression thus enabling calls between these two environments. VoIP gateway carries out its function in accordance with recommendations of the ITU-T for multimedia communication H.323v3, version 3. Recommendation H. 323, unlike the first version, enables a definition of instruments of quality of service. Cisco voice gateway provides connection to public telecommunication network. On this gateway, signals from packet network are transformed into signals for a network which operates on the principle of switching circuits. On a signalisation level, signalisation messages are being mutually mapped between the H.323 protocol and other protocols such as DSS1, Q.SIG etc.

Voice gateways are usually routers equipped with the relevant interfaces (BRI, PRI) and instruments for digital processing of the signal (so-called DSP). Most frequently used are Integrated Services Routers (ISR) of platforms 26xx, 28xx, 38xx.