How to know whether I am in a call on Android?

TelephonyManager.getCallState() returns one of

• CALL_STATE_IDLE
• CALL_STATE_OFFHOOK
• CALL_STATE_RINGING

If this fits your requirements, it's much less code than Pied Piper's more comprehensive solution.

You need broadcast receiver ...

In manifest declare broadcast receiver ...

<receiver android:name=".PhoneStateBroadcastReceiver">
        <intent-filter>
                <action android:name="android.intent.action.PHONE_STATE"/>     
        </intent-filter>
</receiver>

Also declare uses-permission ...

<uses-permission android:name="android.permission.READ_PHONE_STATE" />  

The broadcast receiver class ...

package x.y;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.telephony.PhoneStateListener;
import android.telephony.TelephonyManager;

public class PhoneStateBroadcastReceiver extends BroadcastReceiver{

    @Override
    public void onReceive(Context context, Intent intent) {

        TelephonyManager telephonyManager = (TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);
        telephonyManager.listen(new CustomPhoneStateListener(context), PhoneStateListener.LISTEN_CALL_STATE);

    }

}

And one class to customize phone state listener...

package x.y;
import android.content.Context;
import android.telephony.PhoneStateListener;
import android.telephony.TelephonyManager;

public class CustomPhoneStateListener extends PhoneStateListener {

    //private static final String TAG = "PhoneStateChanged";
    Context context; //Context to make Toast if required 
    public CustomPhoneStateListener(Context context) {
        super();
        this.context = context;
    }

    @Override
    public void onCallStateChanged(int state, String incomingNumber) {
        super.onCallStateChanged(state, incomingNumber);

        switch (state) {
        case TelephonyManager.CALL_STATE_IDLE:
            //when Idle i.e no call
            Toast.makeText(context, "Phone state Idle", Toast.LENGTH_LONG).show();
            break;
        case TelephonyManager.CALL_STATE_OFFHOOK:
            //when Off hook i.e in call
            //Make intent and start your service here
            Toast.makeText(context, "Phone state Off hook", Toast.LENGTH_LONG).show();
            break;
        case TelephonyManager.CALL_STATE_RINGING:
            //when Ringing
            Toast.makeText(context, "Phone state Ringing", Toast.LENGTH_LONG).show();
            break;
        default:
            break;
        }
    }
}