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Glossary · Voice technology

What is Ringback tone?

Reviewed by Sujan ThapaliyaLast updated

Definition

A ringback tone is the sound the caller hears while the far end is ringing. It is generated by the network, not by the recipient's phone, which is why the number of rings you hear does not match the number the recipient hears.
The three STIR/SHAKEN attestation levels, which underpin how ringback tone behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, ringback tone included.

In practice

How ringback tone actually works

Because the tone is network-generated, hearing one ring before voicemail does not reliably prove you were blocked; the two events are only loosely coupled.

Ringback patterns differ by country and are a quick way to tell where a call actually terminated: the UK's double ring is unmistakable next to the US single tone.

Ringback is also why call progress cannot be trusted as evidence. Some routes generate it locally as soon as setup begins, so a caller can hear thirty seconds of ringing on a call the far end never received. When diagnosing unanswered calls, the carrier's signalling record is the only reliable account of what happened.

Worked example

A caller in London hears the two-tone British ring while the same call to a US number rings once, long. Neither says anything about whether the distant handset is actually alerting.

Ringback tone: common questions

Why do I hear ringing before the phone actually rings?
Because the ringback you hear is generated by the network, not by the distant handset. It starts as soon as the call is being set up, which is why a call can be answered before you have heard a full ring, or ring on your end while nothing rings at theirs.
Does the ringback tone tell me the call is connecting?
Only that setup is progressing. On some routes ringback is played while the call is still being placed onward, which is why a long ringback with no answer is not proof the destination handset was ever alerted.
Why does ringback sound different in other countries?
Because cadence and frequency are national conventions. The two-tone British ring, the single long American ring and the continental patterns are all normal, which matters when diagnosing international calls that sound wrong but are working.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

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