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Telecom and SIP Glossary: Phone Numbers, SMS and VoIP Terms

Updated 2026-09-30 · By the ZuteSIP team (ZuteSIP Inc.)

Short answer: This glossary explains the telecom, SIP and verification terms you meet when buying and using a phone number online. Each entry gives a one-sentence definition, how the technology works, and what it means in practice for a ZuteSIP number, which is a real mobile, local or toll-free number with SIP credentials included.

Telecom vocabulary is full of acronyms that vendors use loosely. These pages define each term precisely, point to the standard behind it where one exists, and link to the guides where you can put it to use.

  • SIP — The signalling protocol that sets up, manages and ends internet voice calls.
  • SIP trunk — A SIP connection that carries calls between a phone system and the phone network.
  • DID — Direct inward dialing: a number that routes straight to a specific line or account.
  • Softphone — Software that makes and receives phone calls on a computer or phone over SIP.
  • PBX — A private branch exchange: the phone system that routes calls inside an organisation.
  • Web dialer — A dialer that runs in a web browser, so you can call without installing software.
  • E.164 — The ITU-T international format for phone numbers: + country code and subscriber number.
  • Toll-free number — A number the caller dials without being charged for the call.
  • Local number — A geographic number tied to a city, region or area code.
  • Mobile number — A number from a country's mobile range, typically able to receive SMS.
  • Virtual number — A phone number used over the internet rather than tied to a physical SIM or line.
  • Non-VoIP number — A number that carrier databases classify as mobile or landline rather than VoIP.
  • OTP — A one-time password, usually a short code sent by SMS or call to confirm identity.
  • Two-factor authentication — Sign-in that needs a second proof, such as a code, on top of a password.
  • SMS verification — Confirming a phone number by sending a code to it in a text message.
  • Call forwarding — Automatically sending incoming calls on one number to another phone.
  • Caller ID — The number shown to the person you call when your call reaches them.
  • G.711 codec — The uncompressed ITU-T voice codec with µ-law (PCMU) and A-law (PCMA) variants.
  • DTMF — The tones sent when you press keypad digits during a call, used by menus and PINs.
  • NAT and STUN — How routers rewrite addresses, and the protocol SIP devices use to cope with it.
  • IVR — Interactive voice response: an automated phone menu driven by keypresses or speech.
  • CNAM — Caller name delivery: the name shown next to a number on some networks.
  • STIR/SHAKEN — A framework that signs and verifies caller ID to fight spoofed robocalls.
  • Number porting — Moving an existing phone number from one provider to another.

How to use this glossary

Terms are grouped loosely into three families: number types (DID, local, mobile, toll-free, E.164), calling technology (SIP, SIP trunk, softphone, PBX, codecs, DTMF, NAT) and verification and identity (OTP, 2FA, SMS verification, caller ID, CNAM, STIR/SHAKEN).

Every term page ends with a short section on how the concept applies to ZuteSIP, so you can see which features are part of the service and which are not.

Number types at a glance

TermWhat it describes
Mobile numberA number from a country's mobile numbering range
Local numberA geographic number tied to a city or area code
Toll-free numberA number where the caller is not charged for the call
DIDAny number that routes straight to a line, extension or SIP account
E.164The international format that writes any of them as +country code and number

Where ZuteSIP fits

ZuteSIP supplies real phone numbers in 181 countries, each with unlimited incoming SMS and calls, 500 outgoing SMS and 1,000 outgoing minutes per month, per-number SIP credentials, call forwarding and a browser web dialer. Every number type costs the same, from $1.80 per month billed yearly ($21.60 per year).

Some glossary terms describe features that are not part of the service, such as number porting or call recording. Those pages say so plainly.

Standards referenced

  • IETF RFC 3261 for SIP signalling
  • ITU-T Recommendation E.164 for international number format
  • IETF RFC 4733 (successor to RFC 2833) for DTMF sent as RTP events
  • ITU-T G.711 for the µ-law and A-law voice codecs

Next steps

If you are new to SIP, start with the SIP, softphone and DID entries, then follow the setup guide to connect a number to your first app.

Frequently asked questions

Who is this glossary for?

Anyone buying or using a phone number online, from people receiving verification codes to teams connecting numbers to a PBX. Each entry starts with a plain definition before the technical detail.

Are ZuteSIP numbers real phone numbers?

Yes. They are real mobile, local and toll-free numbers on national phone networks in 181 countries, often called virtual numbers because you use them online.

Does every term describe a ZuteSIP feature?

No. Some entries, such as number porting, describe industry concepts that are not part of the service, and those pages say so.

Where do I start if I am new to SIP?

Read the SIP and softphone entries, then follow the guide to setting up a SIP dialer.

Related pages

Sources