Resources
The reference for business voice and messaging.
Clear, factual guides to the concepts behind business calling and texting: inbound and outbound traffic, 10DLC, caller ID, HD Voice, STIR/SHAKEN, and the rest of the stack.
55 guides, grouped by topic.
Voice
How calls move, what HD voice actually is, and the protocols that carry audio.
What is VoIP?VoIP (Voice over Internet Protocol) converts speech into IP packets and sends them over a data network. Signaling and media are separate: a protocol such as SIP sets up the call, and RTP carries the audio.Read guide HD VoiceHD Voice is wideband telephone audio: roughly 50-7000 Hz instead of the classic 300-3400 Hz narrowband channel. Both ends of the call, and every hop between them, must carry a wideband codec without transcoding down to narrowband, or the call falls back to standard narrowband audio.Read guide Inbound trafficInbound traffic is telephone traffic that terminates to a number you publish: customers, vendors, and partners dialing in. In carrier language that direction is often origination: the PSTN originates a call toward your VoIP endpoint.Read guide Outbound trafficOutbound traffic is calls a business places: agents dialing customers, notifications, and transfers off-net. Carriers treat that direction as termination onto the PSTN or a peered IP network.Read guide SIPSIP (Session Initiation Protocol) is the IETF signaling protocol for initiating, modifying, and terminating multimedia sessions. RFC 3261 is the core specification. SIP sets up the call; the voice itself travels over RTP.Read guide WebRTCWebRTC (Web Real-Time Communication) is a set of W3C APIs and IETF protocols that let a browser capture a microphone, negotiate a peer connection, and send encrypted RTP. It is the usual substrate for click-to-call and in-app calling.Read guide PSTNThe PSTN (public switched telephone network) is the worldwide system that routes calls to telephone numbers. It is no longer only analog copper and electromechanical switches, but it remains the numbering and interconnection fabric VoIP depends on to reach anyone with a phone number.Read guide CodecsA codec (coder-decoder) turns speech samples into a compact bitstream and back again. In telephony the choice of codec sets bandwidth, HD capability, DTMF robustness, and how a call sounds after loss.Read guide Call qualityCall quality on VoIP is the listener's experience of delay, distortion, and dropouts. The usual technical drivers are one-way latency, jitter, and packet loss. MOS (ITU-T P.800) is how that experience is scored.Read guide DTMFDTMF (Dual-Tone Multi-Frequency) is the keypad signaling system that sends digits, *, and # as pairs of tones. On VoIP, those events should travel as named RTP events (IETF RFC 4733), not only as audio, or IVR menus will miss them.Read guide Call recordingCall recording stores the audio of a telephone session (and often metadata and transcripts) for quality, dispute, or regulatory reasons. How you mix the channels, when you start, and whether you have consent are the decisions that matter.Read guide VoIP network requirementsA VoIP call needs modest bandwidth but strict delivery: low one-way latency, low jitter, and almost no packet loss, in both directions. Most 'VoIP does not work here' problems are network problems, and most of those are on the upload path or the Wi-Fi hop.Read guide What is a softphone?A softphone is a software application that places and receives calls over VoIP, using a computer, tablet, or mobile phone instead of dedicated telephone hardware. One user and one number can register several endpoints at once, and an inbound call can ring all of them.Read guide
Numbers & identity
Caller ID, CNAM, DIDs, E.164, porting, and the difference between local and toll-free.
Caller IDThe number presented when a call arrives, and why it is not the same as a name lookup, a billing identifier, or a cryptographic attestation.Read guide CNAMCaller Name: a short, unauthenticated string some terminating carriers look up when a NANP number rings.Read guide DIDDirect Inward Dialing: a number that reaches a specific endpoint without a receptionist, from classic PBX trunks to today’s virtual numbers.Read guide E.164The ITU-T international public telecommunication numbering plan: plus, country code, national number, 15 digits maximum.Read guide Number portingLocal Number Portability: moving a telephone number from one carrier to another while keeping the digits the public dials.Read guide Toll-free numbersNon-geographic 8XX numbers the caller does not pay for in the US, managed by a RespOrg in the SMS/800 database.Read guide Local numbersGeographic NANP telephone numbers: area code, exchange, rate center, and why “local” is a rating concept as much as a city name.Read guide ANIAutomatic Number Identification: the billing-grade calling number used by the network, distinct from the Caller ID the handset shows.Read guide What is a shared phone number?A shared phone number is a single business number that several people use at once: inbound calls ring more than one person, outbound calls and texts present the same number, and the conversation history is visible to the whole team. To the network it is one ordinary DID; the sharing is platform behavior.Read guide What is a second phone number?A second phone number is a separate business line that rings an app on the phone you already carry: no second SIM, no second device. The number is an ordinary DID on a VoIP platform; the phone in your pocket is just one of its endpoints.Read guide What is branded calling?Branded calling displays a verified business name, and often a logo and call reason, on the recipient's screen for outbound calls. It is the step past CNAM: instead of an unauthenticated 15-character database dip, the display rides on cryptographic call authentication.Read guide
Messaging
10DLC, A2P, SMS, MMS, short codes, RCS, and the Campaign Registry.
10DLC10-digit long code application-to-person messaging in the United States: a carrier registration regime on local numbers, not a new numbering plan.Read guide A2P messagingApplication-to-person messaging: software sending SMS, MMS, or RCS to people, as distinct from person-to-person texting.Read guide SMSShort Message Service: store-and-forward text over mobile networks, still framed as 140-byte segments with optional concatenation.Read guide MMSMultimedia Messaging Service: images, audio, video, and vCards on a path that is related to SMS but not the same PDU.Read guide Short codesFive- or six-digit numbers leased for high-volume application-to-person messaging, with a long provisioning cycle compared with 10DLC.Read guide Toll-free textingSMS and MMS on 8XX numbers: a verification regime separate from 10DLC, after carriers sunset unregistered toll-free traffic.Read guide RCSRich Communication Services: the GSMA profile for branded, receipted messaging over IP, alongside SMS rather than inside iMessage.Read guide The Campaign RegistryThe industry registry US wireless carriers use for 10DLC: brands, campaigns, CSPs, DCAs, and declared use cases.Read guide Why text messages get filtered or blockedWhere messages disappear between an API call and a handset: delivery receipts and their limits, the three independent filtering layers, and the content, registration, and volume signals that trigger them. Registered 10DLC traffic with clean opt-out hygiene is the baseline, not a guarantee.Read guide SMS consent and opt-outsThe operational layer of texting compliance: which consent tier permits which traffic, what a compliant opt-in contains, and how STOP, HELP, and quiet hours work in practice. The statute itself lives in the TCPA guide; this is the day-to-day hygiene that keeps a program clean.Read guide
Routing
IVR menus, forwarding, hunt groups, and queues: how a call finds a person.
What is IVR?Interactive Voice Response (IVR) is a system that plays prompts, collects keypad or spoken input, and routes or answers a call without a person on the first hop.Read guide Call forwardingCall forwarding is a routing rule that sends an inbound call from one destination (a DID, extension, or user) to another number or device when conditions match.Read guide Hunt groupsA hunt group (also called a ring group in many PBXs) is a list of members and an algorithm that decides who rings when a call arrives for the group.Read guide Call queuesA call queue is a buffer for inbound calls. Automatic Call Distribution (ACD) then assigns the next waiting caller to an available agent according to a routing policy.Read guide Blind vs warm call transfersA transfer moves a live call from the person who answered it to someone else. A blind transfer sends the caller immediately, sight unseen. A warm transfer puts the caller on hold, consults the target first, then hands the call over with context.Read guide Voicemail, transcription, and visual voicemailVoicemail is the terminal state of call routing: where a call lands when forwarding rules, hunt groups, and queue overflow have all run out of people to ring. Modern systems add transcription, email delivery, and list-based visual voicemail on top of the recorded message.Read guide
AI & automation
AI receptionists and the Model Context Protocol: how software answers calls and how agents reach a phone system.
What is an AI receptionist?An AI receptionist is software that answers inbound calls and holds a spoken conversation: it transcribes the caller, decides what to do with a language model, and replies with synthesized speech. Unlike an IVR tree, there is no fixed menu; unlike voicemail, the call is handled, not just recorded.Read guide What is MCP (Model Context Protocol)?The Model Context Protocol (MCP) is an open, JSON-RPC-based standard that lets AI applications discover and call external tools and data sources. Applied to telephony, an MCP server exposes a phone system's actions, calls, messages, numbers, and contacts, so an AI agent can operate them with the user's permission.Read guide Call recordings, transcripts, and AI: where the data goesA modern business phone system holds more than audio: call records, recordings, voicemails, transcripts, message history, and AI-generated summaries. This guide maps what exists, where it lives, and the questions that determine whether an AI feature is safe to turn on.Read guide
Trust & compliance
STIR/SHAKEN, spoofing, TCPA, E911, and how the industry fights robocalls.
STIR/SHAKENSTIR/SHAKEN is the call-authentication framework used on IP voice networks in the United States. The originating provider signs a token that says how much it knows about the calling number. The terminating provider verifies that signature.Read guide Caller ID spoofingCaller ID spoofing means the number shown to the called party is not the true originating line identity. Some substitution is ordinary telephony. Spoofing with intent to defraud, cause harm, or wrongfully obtain value is unlawful in the United States.Read guide TCPAThe Telephone Consumer Protection Act (TCPA), 47 U.S.C. § 227, is the primary U.S. federal statute on autodialed calls, prerecorded voice, marketing consent, and the Do Not Call framework. It includes a private right of action.Read guide E911Enhanced 911 (E911) is 911 service that delivers a callback number and location information to the public safety answering point (PSAP), not only a voice path. On business phone systems, federal MLTS rules add direct dial and dispatchable location duties.Read guide RobocallsIn U.S. telecom usage, "robocall" means an autodialed call or a call that uses an artificial or prerecorded voice. Illegal robocalls are a traceback, blocking, and authentication problem as well as a TCPA problem.Read guide Why calls get labeled Spam Likely"Spam Likely" is a verdict from a carrier's analytics engine, not an entry in a central registry. Legitimate businesses get labeled when their numbers or calling patterns resemble Robocalls, and the label suppresses answer rates whether or not anyone notices it.Read guide VoIP security and encryptionA business phone system faces the same adversaries as any networked service, plus one unique incentive: calls cost money, so a compromised account can be monetized directly. The defenses are an encryption stack (TLS and SRTP), credential hygiene, and fraud controls on what the system is allowed to dial.Read guide
Infrastructure
SIP trunks, CPaaS, and the origination / termination split carriers bill against.
SIP trunkingSIP trunking delivers telephone calls over IP using SIP signaling and RTP media, in place of a dedicated TDM circuit such as a PRI. Capacity is counted in concurrent call paths, not in copper channels.Read guide CPaaSCommunications Platform as a Service (CPaaS) is a cloud platform that exposes programmable voice, messaging, and sometimes video through APIs and SDKs, rather than a full employee phone system or agent desktop.Read guide Call origination and terminationIn carrier billing, origination is placing a call onto the network. Termination is completing it to the called number. Inbound service to a DID is termination to that number from the caller's carrier's point of view.Read guide VoIP reliability and failoverWhat actually happens to calls when the internet, a device, a platform, or a carrier fails. A phone number is a routing entry rather than a wire, so reliability is a question about where calls go next, answered layer by layer with failover forwarding, redundant endpoints, and platform architecture.Read guide PBX, cloud PBX, and UCaaSThe vocabulary of the business phone market, anchored to its history: the PBX as a switchboard the business owned, and every later category as a decision about who runs that switchboard and what else ships with it. Cloud PBX, UCaaS, CCaaS, and CPaaS are neighbors, not synonyms.Read guide