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Legal restrictions

Please note:  legal restrictions in your country may require clear and unambiguous consent for recording by all parties to a call.

Feature Overview

From firmware version and forward, any phone with USB Recording Feature can record a phone call to an attached USB drive in WAV file format. If the setting usb_storage_passphrase is set, the audio will be encrypted while recording.

Audio file encoding

The audio file is recorded using these specifications:

  • 16 bits per sample
  • 16 kHz sample rate
  • Mono channel
  • PCM signed 16-bits little-endian, a.k.a s16l

Playback of WAV files stored on a USB drive is possible provided the file encoding as above and no special characters in the filename. Any of the following special characters included in the filename will prevent the filename from being displayed: comma, /, \, #, ;, :, |, %, (, ), @, &, ?, ', ", *, >, <, and whitespaces.

Required Settings

On a USB Recording Feature supported phone, follow the next steps to be enable recording on a USB drive or playback previously recorded files from an attached USB drive:

  • Attach USB drive to the phone
  • Edit setting phone_features by appending the following feature name: UsbRecordPlayback. See here for instructions on editing the setting.
    The current feature names must be separated by space. Here is an example on how the setting would look like after appending UsbRecordPlayback on a D735 (version

  • Next to "Some settings are not yet stored permanently. " press Save

  • Reboot the phone

Once enabled, two feature keys can be assigned to any programmable key but also will be automatically added to the soft keys when in a call. The feature control keys are Record and Pause.

Recording Operation

With an active call, the typical operation is:

  • Start Recording
  • Pause Recording
  • Resume Recording
  • Stop Recording

Below is a sample of the in-call keys with the USB Recording feature enabled, but not actively recording.

Actively recording will appear similar to the image below.

After pressing Pause, you have the option of Stop or Resume, as shown below.

Determining Available Recording Space

With a phone that has the USB Recording Feature available and enabled (see Required Settings above), navigate to the USB Info page with the following steps:

  • Press the Settings key
  • Scroll to Information, press OK
  • Scroll to USB Info, press OK

Depending on the size of the attached USB drive you will see a screen similar to the image below.

Listing Playback Files

With a phone that has the USB Recording Feature available and enabled (see Required Settings above), navigate to Browser Recordings with the following steps:

  • Press the Settings key
  • Scroll to Call Features, press OK
  • Scroll to Browse Recordings, press OK

A list of WAV files will be displayed similar to the image below

Playback Steps

Following the Listing Playback Files above, select the file you wish to play and press the Play soft key. See a sample image below.

The playback can be paused, deleted, or stopped.
When the file is not encrypted and actively playing Rewind and Fast Forward allow skipping forward or backward by 5 second intervals.
When encrypted and playing Rewind and Fast Forward do not appear, as skipping is not feasible with encrypted audio.


  • The recording is not supported for local (phone MCU) conference (2+1) calls. We suggest using remote conference call (server MCU) instead.
  • Rewind/Forward action is not available when playback encrypted recording
  • Filenames cannot contain whitespace