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To gradually eliminate or remove someone or something. A noun or pronoun can be used between "phase" and "out." Of course I'm worried—the corporate office is phasing out all of the jobs in our department! When was the two-dollar bill phased out of circulation?
To take something or someone out of service gradually or in stages: The company phased out the old model of vacuum cleaner, and it's hard to find any in the stores now. We will gradually phase the old schedule out to create a more efficient one.