Conflict of method call using Interface in Java

Yesterday, I had an interview and I was given the following scenario:

There are 3 classes namely Main.java, MobilePhone.java, DeskPhone.java and one Interface ITelephone.java. powerOn() method is implemented in both classes MobilePhone.java and DeskPhone.java.

How can I call powerOn() method in DeskPhone class after creating an instance of MobilePhone class? In other word, how can I print "You are in DeskPhone class" and "You are in MobilePhone class" in last two calls in Main class?

Is there any another way to solve this problem without renaming powerOn() method in either of class?

Main.java

public class Main {

public static void main(String[] args) {
    ITelephone timsPhone;
    timsPhone = new DeskPhone(123456);
    timsPhone.powerOn();
    timsPhone = new MobilePhone(45678);
   //Question begins here 
    timsPhone.powerOn();
    timsPhone.powerOn();

    }
} 

ITelephone.java

public interface ITelephone {
     void powerOn();
}

MobilePhone.java

public class MobilePhone implements ITelephone{

    private int myNumber;

    public MobilePhone(int myNumber) {
        this.myNumber = myNumber;
    }
    @Override
    public void powerOn() {
        System.out.println("You are in MobilePhone class");
    }
}

DeskPhone.java

public class DeskPhone implements ITelephone {

        private int myNumber;

        public DeskPhone(int myNumber) {
            this.myNumber = myNumber;
        }

        @Override
        public void powerOn() {
            System.out.println("You are in DeskPhone class");
        }
}