Pros:
- no need to purchase separate device
- no need to take care of extra batteries in extra device

Cons:
- when smartphone is needed, it is busy with other task (recoding) > may lead to unexpected situations

Task:
- record voice track during long period of time
- easily transfer files from phone

Requirements to meet:
- free (going on the budget)
- record in background
- work with attached external microphone
- WAV is preferable, but not mandatory

There are a lot of applications. Here is a list of free voice (audio) recorders available for Android OS:
Remark: In this list covered functionalities of free versions, but no Purchase-In/paid versions of them.
- Voice Recorder (blue icon)
    /storage/emulated/0/vrecorder/
- Voice Recorder (dark-red icon)
   ?
- Easy Voice Recorder
    /storage/emulated/0/EasyVoiceRecorder
- Voice Recorder (red icon, mic sliced)
    - permissions to call
    - asked for phone permissions and refused to record without it (with does Voice Recorder has to have rights to be able to record from microphone? if I am not recording phone call?)
    ! to uninstall
- Smart Recorder (with red dot)
    ?
- ASR Recorder
    ?
- REV Recorder
    ?
- Dolby On
    ?
- Music Maker JAM
    for other purposes
- RecForge II
    ?
- Hi-Q Recorder
    /storage/emulated/0/recordings/
- Voice Recorder (red icon with sliced half, haircomb)
    /storage/emulated/0/Android/Data/com.wachi.recorder/files/Music/records


Books:
- [Anton's bookshelf](https://og2k.com/books/)