Thanks to Milan G, he reported all recorders in the market don't work for ZTE Axon Mini. You should root the phone to fix the problem. Even if you have rooted the phone you'll find only Boldbeast Recorder works great, all other recorders still don't work.
After root the phone please use these settings in Boldbeast Recorder to record calls:
- Record Mode: Alsa
- Alsa Device: Auto
- Alsa Bits: Auto
- Alsa Channels: Auto
- Alsa Sampling Rate: Auto
- Fix Recording Issues - Change Audio Controls: Yes
- Fix Recording Issues - Change Audio Driver: No
- Fix Recording Issues - Start Input Stream: Yes
It records calls loud and clear in two sides.