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To be notified of incoming calls in the background using Flutter and Kotlin, you can follow the following steps:

1. Create a Flutter project and add Kotlin support to it.
2. Create a Kotlin class that extends BroadcastReceiver.
3. Override the onReceive() function of the BroadcastReceiver class.
4. Register the BroadcastReceiver class in the AndroidManifest.xml file.
5. Create a Flutter plugin that calls the Kotlin BroadcastReceiver class.

Here is the sample code for the Kotlin BroadcastReceiver class:

class CallReceiver: BroadcastReceiver() {
    private var callStateListener: PhoneStateListener? = null
    private lateinit var telephonyManager: TelephonyManager

    override fun onReceive(context: Context?, intent: Intent?) {
        if (intent?.action == TelephonyManager.ACTION_PHONE_STATE_CHANGED) {
            val state = intent.getStringExtra(TelephonyManager.EXTRA_STATE)
            when (state) {
                TelephonyManager.EXTRA_STATE_RINGING -> {
                    val incomingNumber = intent.getStringExtra(TelephonyManager.EXTRA_INCOMING_NUMBER)
                    // Do something with the incoming number
                }
            }
        }
    }

    fun startListening(context: Context) {
        callStateListener = object : PhoneStateListener() {
            override fun onCallStateChanged(state: Int, phoneNumber: String?) {
                if (state == TelephonyManager.CALL_STATE_RINGING) {
                    // Do something with the incoming number
                }
            }
        }
        telephonyManager = context.getSystemService(Context.TELEPHONY_SERVICE) as TelephonyManager
        telephonyManager.listen(callStateListener, PhoneStateListener.LISTEN_CALL_STATE)
    }

    fun stopListening() {
        telephonyManager.listen(callStateListener, PhoneStateListener.LISTEN_NONE)
    }
}

In your Flutter plugin, you can create methods that call the startListening() and stopListening() functions of the CallReceiver class to start and stop listening for incoming calls. You can use MethodChannels to communicate between Flutter and the native platform.

With this approach, you can receive notifications of incoming calls even when your app is in the background.