I find that usually on my game I get calls if I'm outside and the time is a five miniute mark (five past, ten past, quarter past etc.) and I walk one step. Also people call you almost immedeatly after you turn the game on. I'm not sure if more calls mean more rematches though.
This also works in R/S: If you keep on going in and out of a route the peaple on that route will want to rematch you quicker. (I used this trick with route 117 and mauville)