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.

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

VoIP Security
Security in VoIP has not been a very popular subject so far, but with the increasing demand for VoIP, it is becoming an important concern. VoIP security threats are increasing in number.
VoIP Security Threats - Making Your VoIP Secure
VoIP Phishing - What It Is And How It Works
Do Softphones Make My Network or Machine More Vulnerable? Using Firewalls

For phone users who want a type of VoIP is more or less as their existing fixed line with traditional phones, the most appropriate types of VoIP services are subscription and equipment services. They are intended mainly to households and small businesses. Upon subscription, the user is sent an adapter (ATA) and is given a phone number. This requires an Internet connection, which adds cost, but the overall cost is relatively much weaker. Added benefits are the many features that come with VoIP.
Factors Affecting VoIP Voice Quality VoIP Drawbacks Running VoIP on a Wireless LAN