The last step of Factory reset which you have already tried.
If its on voice call maybe if you are near to any other device the echo can be heard . Make sure you are not near to any device which is on.
Also, have you tried with a different SIM?
If the issue still persists. I would recommend you to visit service center:
Kindly visit the nearest service center:
Due to the lockdown, service centers may not be functional. Kindly call the service center before you visit one.https://www.asus.com/in/support/service-center/india