this annoying trouble caused by Zenfone 5 latest system update 2.22.40.53. ASUS team are actively working for fix. for temporary way you can follow this step to fix this issue:
- Turn on Dial pad touch tones (settings>sound) if doesnt work,
- When accept incoming call press "hold" button once (upper right of 9 buttons)
- Then immediatelly press it again, and then you can successfully talk on your phone
This issue only happens on the incoming call