Call audio reliability testing setup

Reliability of phone call audio is crucial for daily driving a smartphone.

Work is being done by @anjandev and @f-izzo

We want to have a hardware/software setup that enables us to make test phone calls and ensure that the audio is correctly working and has a sufficient souund quality. This is done by looping back the target phone earphone output to its microphone input, such that the device making the phone call has can send test samples and record and analyze the received audio.

The proposed hardware setup is composed of:

• Auxiliary device with cellular modem and ability to route call audio to userspace ([generic-zhihe](https://wiki.postmarketos.org/wiki/Zhihe_series_LTE_dongles_(generic-zhihe) is a good candidate)
• Target device with cellular modem and phone call support.
• Headset set up to loopback the target device earphone output to its microphone input

The proposed software setup is the following:

• Route phone call audio to userspace (can be based on the modem distro)
• Play target device headset mic audio to headset headphone to enable audio loopback
• Call audio test software:
  • Make a phone call to target device
  • Send a test audio sample recorded or generated
  • Record the feedback received from target device
  • Analyze the feedback to measure audio functionality and quality

Auxiliary device setup

This device need to be able to make multiple phone calls, so depending on the geographic region we can use:

• XMPP to Call bridge via internet, like JMP Chat
• SIM card with call plan on modem enabled device