Simple VoIP requires very little equipment. More advanced communication Voice over IP requires more sophisticated equipment. What is the equipment for VoIP?
Common VoIP Hardware Devices IP Phones - Special Phones for VoIP What is an ATA? What The Lights On Your Phone Adapter (ATA) Mean What is a Router? - The Functions of a PBX Vendor-Provided Equipment

With VoIP, you use an IP network through an ADSL Internet connection or other, to make or receive calls to / from terrestrial PSTN. You can even talk to people with a landline number using your computer, for example using the SkypeIn and SkypeOut service. PSTN includes telephone numbers and IP networks have IP addresses. How these two work together?
Answer: The answer is address translation. Your computer or VoIP device connected to the Internet has an IP address assigned to it. When you call a person on a fixed PSTN phone, your call is forwarded to the PSTN network via the telephone number of the person you call. And vice versa.