Learning Center: Voice Infrastructure Explained

Phone Numbers and DIDs

Where phone numbers come from, what they can and cannot do, and how one gets from a national numbering plan to your platform. Formats, provisioning, porting, and the rules that govern each.

What is a DID Learn how DID numbers route incoming calls directly to users, systems, or applications. Virtual phone numbers explained Learn how virtual phone numbers work, their main types, uses, and regulations. Number porting: how it works Learn how number porting works, including routing, timescales, and common reasons for rejection. E.164 explained: the international number format Learn how the E.164 standard structures and formats international phone numbers. HLR lookup explained: what it tells you about a number Learn what an HLR lookup returns, including whether a number is live, which network holds it, and whether it has been ported.

AI voice agents

What changes when the thing on your end of the call is a model, not a person. Latency budgets get tight and the failure modes are new.

How AI voice agents connect to the phone network Learn how AI voice agents use phone numbers, SIP trunks, and carrier infrastructure to make and receive calls. Connecting AI voice platforms via SIP: Vapi, Retell, LiveKit, ElevenLabs Learn how to connect Vapi, Retell, LiveKit, and ElevenLabs to the phone network using SIP trunks and DID numbers. What Causes Latency in AI Voice Agents Learn what causes latency in AI voice agents and how to reduce delays across the AI pipeline and telecom network. Getting phone numbers for AI agents in multiple countries Learn how to obtain, connect, and manage local phone numbers for AI voice agents across multiple countries.

Telecom Fundamentals

The vocabulary and the plumbing underneath every voice call. What the PSTN actually is, how a call is signalled and carried, and which layer is responsible for what.

What Is SIP? Learn how SIP sets up, modifies, and ends voice calls, and why signalling is separate from media. What is the PSTN? The fixed telephone network that phone numbers were built for: how calls are switched and routed, how it meets SIP networks, and what survives the switch to all-IP.

SIP Trunking and Connectivity

How a phone system, platform, or application gets a working path to the public phone network. Trunks, authentication, concurrent capacity, and what happens when a route fails.

What is SIP Trunking? Learn how SIP trunks connect a phone system to the public network, and what a trunk actually carries. SIP response codes explained A complete, standards-checked reference to every registered SIP response code, plus the Q.850 mappings and the code confusions that cause most misdiagnosed calls.

Voice Quality and Codecs

Why calls sound the way they do. Codec choice, bandwidth, packet loss, jitter, and the trade-offs between audio quality and network cost.

Voice Codecs Explained: G.711, G.729, Opus, and HD Voice Learn how the main voice codecs differ in bandwidth, audio quality, and where each one is normally used. Latency, jitter and packet loss: what degrades a VoIP call Delay, jitter and loss explained from the standards up: where each one comes from, how they interact at the jitter buffer, and why a loss percentage alone tells you very little.

Caller ID, Compliance and Fraud

What your number tells the person receiving the call, and who decides whether it gets through. Caller ID rules, reputation and labelling, regulatory obligations, and the fraud patterns that shape both.

Caller ID Reputation and "Spam Likely" Labels Explained Learn how call analytics engines label outbound calls, why numbers get flagged, and what affects reputation.