Glossary Core concepts
What is VoIP?
Also called voice over ip, voice over internet protocol, internet telephony
VoIP, or Voice over Internet Protocol, is the practice of carrying telephone calls as data packets across an IP network instead of over a circuit dedicated to that one call. It is the underlying technique behind essentially every modern business phone product, including SIP trunking and hosted phone systems.
How a call actually travels
Your voice is sampled, compressed by a codec, and cut into small packets. Each packet is addressed and sent across the network independently, which means packets can take different routes and arrive out of order or not at all. The receiving end reassembles them, smooths out the timing differences with a jitter buffer, and plays the result. This is why VoIP quality is a network question rather than a phone question: the handset is rarely the problem.
VoIP is a technique, not a product
People often use "VoIP" to mean a cheap phone service, which muddles the conversation. It is more useful to treat VoIP as the transport and then be specific about what is being bought on top of it: SIP trunking is VoIP delivered to a phone system you own, a hosted phone system is VoIP plus the phone system itself, and a softphone app is VoIP with the handset replaced by software.
What to fix first when VoIP sounds bad
Almost every "the VoIP is bad" complaint traces to the local network rather than the carrier. Check in this order: test from a wired connection to rule out wifi, look at whether the uplink is saturated at the times people complain, and confirm the router is not doing SIP ALG, a feature that claims to help VoIP and frequently mangles it. Only after those three should you suspect the provider. The reason for that order is cost: the first three are free to check and account for the overwhelming majority of cases.