VoIP (Voice over IP)

VoIP, short for Voice over Internet Protocol, refers to the family of technologies that carry voice communication — and, by extension, SMS and other related messaging — over IP-based packet networks rather than traditional circuit-switched telephone networks. Where a classical phone call would have reserved a dedicated 64 kbps channel through the PSTN for its duration, a VoIP call digitizes the voice signal, compresses it with a codec, packetizes it, and sends it as RTP (Real-time Transport Protocol) packets over an IP network.

VoIP is built on a stack of protocols. SIP (Session Initiation Protocol, RFC 3261) handles call setup, teardown, and signaling between endpoints. RTP carries the actual media payload. Codecs such as G.711, G.729, Opus, and AMR-WB compress the audio to fit reasonable bandwidth budgets. Variants like WebRTC stack additional browser-friendly transport (ICE, STUN, TURN, DTLS-SRTP) on top of similar primitives.

VoIP is the underlying transport for the modern telephony ecosystem far beyond what most users see. Major carriers run VoIP between their core network elements; office PBX systems are virtually all VoIP-based; consumer-facing apps such as WhatsApp, Telegram, Signal, FaceTime, and Discord use VoIP-style media transport for voice and video. Even calls that originate or terminate on a traditional handset typically traverse VoIP segments along the way.

In the context of phone numbers and verification, VoIP is significant because VoIP-classified numbers behave differently from real-SIM mobile numbers in fraud-detection systems. Number-intelligence services (Telesign, Twilio Lookup, Sinch) tag numbers as one of several types — mobile, landline, fixed-VoIP, non-fixed-VoIP — and downstream services use these tags to decide whether to accept the number. Most virtual phone numbers used for SMS verification are classified as non-fixed-VoIP, which causes some strict services to reject them.

The distinction between fixed and non-fixed VoIP is regulatory rather than technical. Fixed-VoIP numbers are tied to a registered service address (typically a residential or business location), comply with E911 location-routing rules, and look more like landlines to fraud systems. Non-fixed-VoIP numbers are not tied to a physical address and are used by services that need numbers without provisioning physical lines per customer.

VoIP has effectively replaced traditional circuit-switched telephony for new deployments. The transition has been ongoing since the early 2000s and is largely complete in most of the developed world; the remaining circuit-switched infrastructure is in regulatory and legacy edge cases.