Speech Reinforcement: Telephone Output

By: Owen T. Heisler
Updated: 2021-09-28
Published: 2016-08-05

This article is part of a series; see Speech Reinforcement.

Sometimes a phone and a conference calling service are used to share the audio with remote listeners.

Frequency ranges

The human speech critical frequency range is approximately 170 Hz to 8.3 kHz (see Human Speaker).

However, in the United States, the reproduced frequency range on a typical phone service on the PSTN (publicly-switched telephone network) is approximately 300 Hz to 3.4 kHz. This is not adequate for high-fidelity speech reproduction; both the fundamental frequency of most speech and much of the sibilance of speech falls outside of this range.

Connecting to a phone input

I know of a few products that may be helpful for this. I recommend using a simple wired connection that does not require any power, if possible. However, there are some options I know of that use Bluetooth.

kV Connection KM-IPHONE-MICX*

kV Connection sells two XLR to TRRS adapters for connecting a system output to a cell phone’s microphone input: the KM-IPHONE-MICX and the KM-IPHONE-MICX-A22. These products are identical except that the second one attenuates the signal by 25 dB.

Most outputs will be line level rather than microphone level, and in that case the KM-IPHONE-MICX-A22 is the appropriate option. If the signal level is still too high, or to use the KM-IPHONE-MICX with a line level output, add the Audio-Technica AT8202 adjustable in-line attenuator (selectable 10, 20, or 30 dB attenuation).

As of 2019-05-14, the kvconnection.com website is unavailable. The kV Connection products may no longer be available. If you have any more information, please let me know.


Brian Jagielski recommended this Bluetooth audio receiver with microphone input, powered by micro USB. It is essentially a Bluetooth chip mounted on a breakout board.

This device would be used by connecting a microphone-level output to the Microphone port. To use a line-level output signal, include an appropriate attenuator in the connection to reduce the signal to microphone level.

JK Audio BlueDriver-F3

This is a small device that bridges XLR and Bluetooth. It has an internal Li-Ion battery that must be charged via a mini USB port. I used this for a while and found that repeated pairing was unreliable and the single button control was not intuitive (did not seem to match the description in the documentation). These are the steps that I wrote for using this device (which probably differ from the product’s documentation)—do these with the USB cable disconnected:

  1. To pair, start with the device off (see step c.). Hold the Connect button. The blue will light start blinking rapidly, then stay on, than blink once. Now release. The blue light should be blinking steadily. Initiate the pairing process from the other device. Once paired, the device will be ready for use.

  2. To turn on, hold the Connect button. Wait until the blue light starts blinking rapidly, then release.

  3. To turn off, hold the Connect button. The blue light will turn on, blink, and blink again. Now release.

JK Audio Daptor Three

This is similar to the BlueDriver-F3 but supports 2-way audio and uses a standard 9-volt battery.

Using conference calling services

Conference calling services provide an easy way to share audio with listeners. Be sure to consider the following:

  • It is important that participants be muted by the calling service. Often this requires that certain additional star-codes be dialed when the host starts the conference. However, some services do not provide participant muting at all.

  • In most cases, free conference calling services (such as the one below) are free because they route calls through certain rural areas to take advantage of high intercarrier fees, a practice known as traffic pumping. While it can be convenient to have a free conference service, this can result in users getting unexpected blocked calls, dropped calls, and usage charges even on plans that are marketed as “unlimited”. See related FCC and T-Mobile pages.

  • If you know of a reasonably-priced conference calling service for high volume (probably priced monthly without a contract), please send me a message.


Dial strings in this section use a p to indicate a pause, sometimes dial string pauses are indicated by commas (,). You should be able to save the entire dial string, including pauses, to your phone’s contacts database. Always listen while your phone is dialing to verify successful connection and mode selection.

  1. Register at FreeConferenceCall.com.

  2. Record your dial-in number, access code, and host PIN.

  3. When dialing in to provide program audio to the conference, enter the access code, turn on presentation mode to mute all guests (*5*5), and disable guest enter/exit tones (*8). The full dial string is:

    <dial-in number> pp <access code># pppp <host PIN># ppp *5 p *5 p *8
  4. Guests can be given the dial-in number and access code; they will be prompted to enter the access code and then press the pound/hash button (#). Here is a dial string for guests:

    <dial-in number> pp <access code>#

Other options

There are also many other ways to implement audio streaming, especially over the internet, such as with Asterisk and MusicOnHold streaming. The Opus audio codec would work nicely for this and the audio quality would be much better than what is provided by the regular telephone network.