And that happened about 4 more times, one of those time she picked up the phone and asked why I called her. I said I didn't call her. I received an incoming call from her. And after that until now I keep receiving a notification saying "new voicemail" and there are new voicemail. And these voicemail are the sound mentioned above (the sound you hear when you dial somebody).
So what happened? How can an incoming call when accepted call back? And who made the calls? It appeared that my friend had access to her phone, not like it was stolen or something.