If you are not able to make calls using Wi-Fi or Carrier data you will see a gear moving at the top right corner in the app or “Registering Error” at the bottom of the app. 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. (On an iPhone or iPad, simply double click the home button, locate the running app and swipe it upwards. For ending the app on an Android device, press on the home key until the screens for the running apps are shown. Then swipe the app screen to the right to end the app.)
A rare connectivity issue is found due to port block or firewall set by your carrier. Wi-Fi too gets its internet via carrier so if a carrier has blocked the ports or put firewall, you will get an error using iPlum. 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 iTunes or Google play.
Note for corporate users: If you have Cisco Firewall ASA 5505 family, make sure to switch off sip_policy. Configuration >> Firewall >> Service Policy Rules >> Inspection Default >> edit > Rules actions (tab) > Protocol Inspections (tab) > Sip (Uncheck it)