Please read below to stop it.
You have setup Voice network mode which is a unique feature at iPlum. It helps in getting carrier class voice quality for your iPlum calls without using mobile data.
For incoming calls in voice network mode, you will first see a notification alert on your phone and then the actual ringing screen.
Accept the call by sliding on screen or push green button.
If you do not want to take the call, do not click the “red decline” button. Let it ring and it will go to your iPlum voicemail. If you need to stop the ringing tone, gently push the side power button on your device. The ringing will stop immediately and after 15/20 seconds (ringing time), iPlum will route the unanswered call to your iPlum voicemail. If you tap “red decline” button on the ringing screen, your device will force the call to its native voicemail which is inherent hardware limitation.
If you happen to setup Do-Not-Disturb on your device settings, remember to set Do-Not-Disturb on your iPlum settings as well which will route the call to iPlum voicemail before sending it to your voice network.
Advanced Setting : “Use detection to avoid calls going to personal voicemail”. You can enable it. It will detect physical voicemail & reroute the unanswered call to your iPlum voicemail.
Disable voice network mode & instead use wi-fi or carrier data for incoming calls?
If Voice Network mode is not working for you, you can disable it any time.
iPlum App: iPlum App Settings || Call Mode: Voice Network or VoIP Data || Incoming Calls || Use My Wi-Fi or Carrier Data.
Online web portal: Login https://my.iplum.com/. Click “Calling Network Mode” in left menu || Incoming Calls || Select Line || Use My Wi-Fi or Carrier Data
How to enable voice network mode for incoming calls with distinct ringtone?
iPlum App: iPlum App Settings || Call Mode: Voice Network or VoIP Data || Incoming Calls || Use My Voice Network. Enter your mobile phone number. Click “Save & Set Distinct Ringtone” || Ringtone: Change from default to a new Ringtone || Done.
Note: Above step may slightly vary for iOS and Android devices.