Trying to apply Progressive Disclosure or Responsive Enabling design pattern

I've been working with a Search filter modal window.

The requirements is to have have the optional fields (IE: Time Range and Call Duration) be disabled or enabled if the user wish to use them as an added search field.

Below is my initial mock up, and tested it with a few people and they're having struggles figuring out that the two disabled fields are optional.

My question is, from my initial mock up do I still need to add visual cues to emphasize that the 2 fields are optional or extra search fields?