Learning center

Voice infrastructure, explained

Resources on voice infrastructure and how phone calls work, from SIP to AI voice agents.

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.
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. 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. Virtual phone numbers explained Learn how virtual phone numbers work, their main types, uses, and regulations. What is a DID Learn how DID numbers route incoming calls directly to users, systems, or applications.

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.
AI cold calling: how it works and what the phone network requires How outbound AI calling actually works on the network: what happens at the moment of answer, why the first two seconds are a regulatory constraint, and what carriers require before your calls get delivered. 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. 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. 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. 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.

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 SIP sets up, changes and ends calls, and then gets out of the way while RTP carries the audio. The protocol's five jobs, the INVITE to BYE handshake, what a SIP message actually contains, and the distinctions that cause most misdiagnosed calls. What is RTP? How voice media travels RTP explained: what the protocol does, what is inside an RTP packet, how payload types and timestamps work, how the media path differs from the signalling path, and what RTCP and SRTP add. What is the PSTN? What defines the public switched telephone network, how PSTN, PLMN and VoIP differ, and why the switch to all-IP changes the transport but not the numbers.

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.
SIP ALG: why it breaks SIP calls and how to diagnose it Learn why the SIP helper built into most routers causes one-way audio, dropped calls and failed registrations, how to confirm it, and what to use instead. 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. What is SIP trunking Learn how SIP trunks connect a phone system to the public network, and what a trunk actually carries.

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.
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. 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.

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.
UAE caller ID allowlisting: why +971 numbers get suspended Learn how UAE caller ID allowlisting works, why +971 caller IDs get suspended, and what keeps a number on the list. VoIP toll fraud and SIP security: how attacks happen Why someone wants your SIP trunk, how they find it, and what a toll fraud incident looks like from reconnaissance to payout. Plus the controls that break each stage, ordered by how early they act. Caller ID Reputation and "Spam Likely" Labels Explained Learn how call analytics engines label outbound calls, why numbers get flagged, and what affects reputation.

Telephony and Contact Centers

What happens to a call once it's answered: queued, routed, and logged. CDRs, PBX design, and the reporting billing and staffing depend on.
Call Detail Records (CDRs): Fields, Examples, and How to Read Them Learn what a call detail record contains, how to read duration, billsec, and disposition fields, and why a single call can produce more than one CDR.

SMS and A2P Messaging

How application traffic reaches a handset, and why so much of it does not. Routing, sender IDs, encoding, delivery receipts, and the carrier rules that decide whether a message is delivered, filtered, or silently dropped.
SMS Character Limits and Concatenation Explained Learn why a long text message becomes several SMS segments, how GSM-7 and Unicode encoding change the character limit, and why segments rather than messages are what you pay for. Alphanumeric sender IDs: rules and registration by country Who has to register a branded SMS sender ID, what the registries ask for, and what happens to unregistered senders. What Is A2P Messaging? What application-to-person messaging actually is, why the SMS standards never define it, and how the terminating network decides whether your traffic gets delivered or filtered.