When you are above 18,000' and ATC lets you know about crossing traffic coming up, if you don't see the traffic right away, there is absolutely no need to call ATC back a few minutes later clogging up the frequency telling ATC that you now see the traffic. Did he ask you to report the traffic? No. It is simply done as a courtesy so you won't be startled when you see the traffic. Many years ago when jets first started flying, pilots almost climbed and descended on their own before warnings were issued. Anyway, no need to be a hero and call back ATC simply to report "traffic in sight", the controller really doesn't care and does not need you to report the traffic, so please stop adding to radio clutter.
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