Piku Zheng
Support documents are very helpful. But I believe the app use IP address to detect if it is on public network or not, which is not accurately everytime, especially working with OpenVPN which has a public IP as privace.
If you are experiencing issues with the audio, please follow the instructions in our guide here:
https://www.zoiper.com/en/support/home/article/14/Missing_or_one_way_audio#android