Why did Desmond call Jack?

In "What They Died For" Desmond called Jack posing as an Oceanic employee and tells him that they found his fathers body.

But why? haha. I know he's trying to get all of them together, but that didn't even lead jack to the concert or anything. Plus Jack was already going with his son, so why bother calling him? I refuse to believe Desmond just prank called Jack. Did I just miss something? hah

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