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Fact sheet · updated

WHAT THE RESEARCH SAYS

Your voice,
minus the top end.

A classic phone call leaves out part of your voice.

Check a call
A person seen from behind at a bright window, holding a smartphone to their ear and looking out over rooftops
AI-generated image, not a photograph of a real person or place.

3,400 Hzthe old ceiling

For decades the telephone has carried speech between about 300 and 3,400 Hz. That is enough to understand most words, but it drops higher sounds that help tell consonants apart. Wideband or HD voice roughly doubles the top of the range.

At a glance

3.4 kHz

Top of the standard telephone band

The standard telephone band runs from 300 to 3,400 Hz. In the 2009 study, narrowband test speech was filtered to that band and sampled at 8 kHz.

NARROWBAND · LIU 2009

7 kHz

Audio carried by wideband coding

ITU-T Recommendation G.722 is titled 7 kHz audio-coding within 64 kbit/s. Its test sequences were published in the Blue Book of 1988.

WIDEBAND · ITU-T G.722

TWO NUMBERS WORTH KNOWING

Narrow band,
wide band.

The top of the classic phone band,
and the top of wideband speech coding.

A 2019 study found that classic narrowband telephone speech is considerably less intelligible than face-to-face speech, partly because of the missing visual cues, the limited bandwidth and background noise.

A QUICK CALL CHECK

How much does
the band matter?

Pick a situation to see what the research says about the narrow telephone band. The bar shows how much the missing high frequencies are likely to matter.

Based on studies in the Journal of the Acoustical Society of America (2009, 2012, 2019) and ITU-T G.722. General information, not medical advice.

Call check

Choose a situation
A wideband HD voice callUp to 7 kHz

Wideband coding such as ITU-T G.722 carries audio up to 7 kHz, so more of the high detail in speech gets through.

A quiet chat on a classic lineUsually fine

Narrowband speech carries most of what is needed in quiet, but it is still missing the information above 3,400 Hz.

Spelling out a name with s and fLose detail

Fricative consonants such as s and f carry useful information above 3,400 Hz, which a narrowband call does not send.

Calling from somewhere noisyBand matters more

In a 2012 conversational study, wideband speech was rated above narrowband, and bandwidth extension helped most in noisy conditions.

Using a cochlear implantOften harder

Implant users generally receive information up to about 8 kHz, so narrowband calls can be an obstacle. Small studies found estimated high frequencies improved understanding.

Sources: Liu and colleagues (2009); Pulakka and colleagues (2012); Nogueira and colleagues (2019); ITU-T Recommendation G.722.

THE SHORT ANSWER

More of the voice
gets through.

Three findings on why calls sound
the way they do, and what helps.

THE OLD BAND

300 to 3,400 Hz

Researchers describe today's standard telephone band as limited to 300 to 3,400 Hz. Anything above it simply is not sent.

Check a call

CONSONANTS

Where the s goes

Information above 3,400 Hz helps identify consonants such as fricatives, like s and f. A wideband codec such as ITU-T G.722 carries audio up to 7 kHz.

See the numbers

NOISE

It matters most in noise

In conversation tests with background noise, wideband speech beat narrowband, and even estimated high frequencies helped.

Clearer calls
A person seen from behind in a busy cafe, holding a phone to one ear and covering the other ear with a hand
AI-generated image, not a photograph of a real person or place.

FOUR THINGS THAT HELP

Clearer calls,
fewer repeats.

These follow from how the phone band works. They will not turn every call into HD voice, but they make the most of what gets through.

  1. Keep your phone up to date

    Ofcom says some 4G devices may need a software or settings update so calls use the right network.

  2. Use a phonetic word for letters

    Because s and f carry detail above 3,400 Hz, saying a word for each letter can help avoid mix-ups.

  3. Step away from noise

    Research found the narrow band hurts most in noise, so a quieter spot gives the call a better chance.

  4. Know it is a known problem

    Studies describe phone calls as a common difficulty for hearing aid and implant users, and researchers are testing ways to restore the missing frequencies.

THE NETWORK IS CHANGING

Old networks out,
new ones in.

Ofcom says the UK's 3G mobile networks are now switched off, and all mobile network operators have confirmed they will switch off 2G by 2033 at the latest. 2G-only devices will need to be replaced with at least 4G.

Ofcom advises checking that any phone you buy, especially from a third-party seller, supports 4G. Your provider should contact you if your device is affected and explain what to do.

An old cream corded telephone handset resting on its cradle on a wooden side table
AI-generated image, not a photograph of a real person or place.

Questions

Questions people ask

Why do phone calls sound thin?

Classic telephone speech is limited to about 300 to 3,400 Hz, so higher frequencies in the voice are not sent. That loses detail that helps tell some consonants apart.

What is HD voice?

It is a name for wideband calling, which carries a wider range of speech frequencies. ITU-T G.722, for example, is a 7 kHz audio coding standard.

Why do s and f get mixed up on the phone?

Information above 3,400 Hz is useful for identifying consonants such as fricatives, and a narrowband call does not carry it.

Does wideband help in noisy places?

In a 2012 conversational study with background noise, wideband speech was preferred to narrowband and to artificially extended narrowband speech.

What has the 3G switch-off got to do with calls?

Ofcom says 3G networks are now off and 2G will go by 2033 at the latest, so older devices need replacing and some 4G phones may need an update to make calls on the right network.

Sources

Where the facts come from

  1. Liu, Fu and Narayanan (2009), Effect of bandwidth extension to telephone speech recognition in cochlear implant users, JASA (PMC)
  2. Pulakka and colleagues (2012), Conversational quality evaluation of artificial bandwidth extension of telephone speech, JASA (abstract, Europe PMC)
  3. Nogueira, Abel and Fingscheidt (2019), Artificial speech bandwidth extension improves telephone speech intelligibility and quality in cochlear implant users, JASA (abstract, Europe PMC)
  4. ITU-T Recommendation G.722: 7 kHz audio-coding within 64 kbit/s
  5. Ofcom: Switching off the UK's 2G and 3G mobile networks, what you need to know

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