I am using a Moto G4 with Zoiper.
Everything worked fine until I upgraded to Android Nougat 7.0
No, when I call XXX, XXX can pick up and hear me but I do not hear anything.
Downgrade to Android 6 is impossible.
You will need to change the audio driver for Zoiper application. In order to do that, open Zoiper, go to "Config" ---> "Audio"---> "Audio driver" and select "External Java Driver". Exit Zoiper and restart your device.
Have the same problem but to change Autiodriver don`t work.
If this issue is related to the Samsung update of Android, it should be resolved in the latest release of Zoiper or in Zoiper Beta. Otherwise you are probably experiencing NAT related issue. For more details regarding the NAT related issues, see here.