Well, the way the program works is that you find Curtis' Xtransciever, and eventually, HE calls YOU. That happens 10 times, and you meet them in front of the Ferris wheel in Nimbasa after arranging it. You talk twenty more times (yeah, I know), and you arrange for another Ferris wheel meeting, which is when you start to trade Pokemon. Since it's Curtis, this is what he trades you:
6th: Shellos (East Sea)
Again, HE will contact YOU, so if you don't get your first phone call by about the 6th or 7th gym, there's a problem.
The most likely places he'll call is Nimbasa, Driftveil, Mistralton, Lacunosa, Undella, Lentimas, and Routes 5,6,7,9,11,12,13,14, 16, which was proven true.
Hope it helps!