Voice over IP or telephony via the Internet, characterizes a set of protocols/technologies [H.323, Session Initiation Protocol (SIP)], which provide vocal conversation in real time with relatively good quality at little or no cost, thanks to worldwide broadband connections. Traditionally, such conversations took place exclusively through a PC which was connected to the Internet and with the aid of a microphone, headphones and the appropriate software (soft phones). The call ended up in another similarly equipped PC at no extra charge other than the one needed to access the Internet, since this specific type of communication does not require a provider of standard land-line services, but only the Internet. In addition to this, there are autonomous telephone devices (VoIP SIP Phones) and analog telephone adaptors (ATA) on the market which directly connect to an IP network, like the Internet. With the right adjustments and assembly and without the need of a PC, use of this service is facilitated making VoIP even more accessible to its users. The object of this paper is the study, realization and operation of an initially autonomous system of telephony via the Internet protocol (VoIP) and subsequently, its connection to other telephony systems (either via the Internet or traditional telephony), for their intercommunication, using the advantages of VoIP. First, the existing phone network [Public Switched Telephone Network (PSTN)] as well as its development is retraced. Then, VoIP technology is analytically presented along with its functions, protocols used, as well as its advantages and disadvantages. The quality of the service (QoS) offered by VoIP and the factors that affect its quality are also studied. As we continue, the importance of IP Private Branch eXchange (PBX) and the additional services they provide will be presented. In conclusion, an application of VoIP for military use will be executed so as to enable the military to reap the benefits it offers.