Can't call non-static method in AndroidJavaClass

I have do the searches for a day and can't find the solution. I want to call a method in an android plugin. I have compiled to jar file and import to Plugins\Android\bin folder, and successfully call a static method to send sms. But I want to get phone's number, so I have to write another method in android java class. Since it's not a static method, I have learned that I have to create an instance of the class to call it. But I can't do that. I have tried several solutions. Here is my code:

 public class SMSActivity extends UnityPlayerActivity {
     
     
     @Override
     public void onCreate(Bundle savedInstanceState) {  
         super.onCreate(savedInstanceState);
     }
     
     
     public String getPhoneNumber() {
         TelephonyManager telemamanger = (TelephonyManager)getSystemService(Context.TELEPHONY_SERVICE);
         return telemamanger.getLine1Number();
     }
 }

in Unity I call the class with:

 ajc = new AndroidJavaClass("com.illidan.sms.SMSActivity");

and create an instance of the class:

 ajo = ajc.Get<AndroidJavaObject>("currentActivity");

But it's not working. The code stops at that line, in logcat I get no static field with name currentActivity. It's really weird because I found some question about this, and according to the replies, that code was working.