Is there any way to mock Singleton object in Scala

I have the following code:

trait Calculator {

  def add(x:Int, y:Int):Int
  def multiply(x:Int,y: Int):Int
}

trait MyCalculator extends Calculator {

  override def add(x: Int, y: Int): Int = x+y //in real live it calls remote service which is not avaialble in test

  override def multiply(x: Int, y: Int): Int = x*y //in real live it calls remote service which is not avaialble in test
}

object MyCalculator extends MyCalculator

Now I have Calculator service:

trait CalculatorServiceTrait {

  def calculate(x:Int,sign:String,y:Int):Int
}

trait CalculatorService extends CalculatorServiceTrait{
  override def calculate(x: Int, sign: String, y: Int): Int = {
    sign match{
      case "+" => MyCalculator.add(x,y)
      case "*" => MyCalculator.multiply(x,y)
      case _ => 0
    }
  }
}

object CalculatorService extends CalculatorService

Now I would like to mock MyCalculator using Mockito to bring me incorrect result.

 "Calculator Service" should{

    "return 0 when 2 and 2 used " in{

      val MyCalculatorMock = mock[MyCalculator]
      when(MyCalculatorMock.multiply(2,2)).thenReturn(0)
      class CalculatorServiceUnderTest extends CalculatorService with MyCalculator

      new CalculatorServiceUnderTest with MyCalculator
      val c = new CalculatorServiceUnderTest
      val result = c.calculate(2,"+",2)
      result shouldEqual(0)
    }
  }

but I'm still getting "4" instead of "0"

Is there any way to handle such test cases?

P.S: I can change some class or trait implementation, but doing global refactoring might be problematic