If you are not able to make calls using Wi-Fi or Carrier data, it is usually due to one of below reasons:
The internet connectivity transition has not occurred on the device. Close and restart the iPlum app.
In certain cases the port block or firewall set by your carrier or wi-fi provider may lead to connectivity issues. Close the app and try to change the source of internet or use a VPN app on your device. There are many VPN apps which you can find in Apple App Store or Google play.
Important: If you are a US or Canada user, you can also use Voice Network Mode for your iPlum calls: