After the call, app connects to number 100000 when using bluetooth in a car

This behavior can occur not only after calls on Ringotel, but also from any other app (for example, WhatsApp). It’s not a bug. It's just a way of the phone communicating with the car's Bluetooth to pass the audio (mic) through.

If the user calls someone via normal cellular service it does not do it. But if the user calls via WhatsApp or Ringotel, the Bluetooth stack in his/her car will dial 100000000 so it can pipe the mic audio through it. It’s not an actual cellular call, but call emulation within the phone’s gsm-module. What the user sees on the car’s display is a notification of this event. It cannot use the usual media channel that it would use for Spotify or Bluetooth audio streaming apps as this does not allow mic (audio) access.

The only way to remove this behavior is to find out the app that's using it and uninstall that app. Specified user has an app installed on their phone that is either sending or receiving audio via Bluetooth. Some users reported, that this could be caused by the Alibaba app, please refer to the following link: https://community.oneplus.com/thread/1521400. We can suggest the user uninstall this app (if it’s installed on the phone) and check whether it helps.