There are 3 ways to place a call in VoIP.
ATA (Analog Telephone Adapter)
Analog Telephone Adapter is a rudimentary form of VoIP hardware. In its crudest form, this device does the process of converting analog to digital signals, and transmits over an internet connection.
IP Phones
These look like regular phones but instead use the RJ-45 Ethernet connection directly instead of RJ-11 connectors. This is the most direct form of using VoIP.
Computer to Computer
If a computer system is equipped with chat microphone, speaker, sound card and an internet connection, with a simple Ethernet/Wireless connection, you can place VoIP calls.