VoIP (Voice over Internet Protocol) is a general term for technology that enables voice calls over internet connections. While traditional PSTN (Public Switched Telephone Network) transmits analog voice signals over copper cables, VoIP converts voice into digital data and sends/receives it as IP packets over the internet. Handling voice as data is common across everything from calling apps to corporate phone systems.
The relationship with IP phones is best framed as technology versus service. VoIP is the general term for the technology, while IP phone is one form of telephone service that uses VoIP technology. 050-number IP phones, Hikari Denwa, LINE calls, FaceTime, and WhatsApp all use VoIP technology. Calls that only connect users of the same app, with no phone number as the destination, are not counted as IP phones; LINE calls are one example.
One reason VoIP is so widely used is that call charges are set differently than on a circuit-switched line. Because voice travels as IP packets, pricing no longer has to follow the idea of occupying a circuit for a given distance and duration, and it is left to each provider to design. Some services charge nothing for calls between users of the same app, while others, such as 050-number IP phones, come with a monthly fee and per-call charges. Rates for international calls are likewise set per destination and per provider, so it is worth checking your provider rate table before you place the call. Businesses combine VoIP with a cloud PBX to carry extension calls between offices over the internet, which keeps the equipment and dedicated trunk lines simpler than installing a switchboard at every site. Calls are controlled via the SIP protocol, with voice compressed and transmitted using codecs.
Disadvantages include call quality being dependent on internet connection quality. Insufficient bandwidth causes packet loss and voice interruptions. High latency disrupts conversation rhythm. QoS configuration that prioritizes voice packets matters here. Additionally, a power outage cuts power to the optical network unit (ONU) and the router, so calls are impossible unless a UPS or similar backup is in place. Emergency calls are limited as well: 050-number IP phones cannot dial 110 or 119. Review technical details in VoIP basics and benefits.