Hi @Absolute_sanjeev, let me see if I can help out here.
If you have a XA1 Plus that has been updated to Android 8 (Oreo), you can find call waiting in the settings of the phone app. In order to activate call waiting, follow the steps below.
- Find and tap Phone > menu button (three vertical dots) > Settings > Calls > Additional settings.
- To activate or deactivate call waiting, tap the switch beside Call waiting.
If you are still running Android 7 (Nougat), the call waiting feature is in the device settings. To enable call waiting, find and tap Settings > Call > Additional settings > mark the Call waiting checkbox.
Let me know if that helped! 
