Maybe you need to do a workaround, let say cover the phonepad by your black-color activity (topmost mode) or right after dialing then you can simulate programmatically a keyhome press, that will hopefully hide the calling phonepad. However if he (the thief) looks at the screen carefully he may still notice that there is an calling icon in the status bar.
Hope this help.
Dim i As Intent
i.Initialize(i.ACTION_MAIN, "")
i.AddCategory("android.intent.category.HOME")
i.Flags = 268435456
StartActivity(i)