I just recently did this sidequest to get a Ralts with Ally Switch, so I'm pretty good at this.
First of all, after getting called by Curtis 10 times in several specific locations, return the Xtransceiver to him. After that, you will be able to call Curtis in any of these locations, but not limited to a specific tile anymore:
Curtis will sometimes not show up on the Xtransceiver, so just try flying to these different places in a loop around the Unova region to get a chance to talk with him 20 more times. From Bulbapedia, I quote:
After the thirtieth call, the player may join Curtis in a ride at the Rondez-View Ferris Wheel. After the first call of every next day, Curtis will ask the player for a Ferris Wheel ride, and if the player has entered the Hall of Fame, he will offer to trade a Pokémon after the ride.
Basically, after meeting Curtis at the Ferris Wheel, you must set the date forward one day and change the time to 23:59 (thanks, Marriland). Go into the game and wait two minutes to start a new day (0:01) and call Curtis from any one of the locations listed above. If you have defeated the Champion, he will want you to meet him once again at the Wheel to trade any Pokemon with him every time you do this. His Pokemon all have their HA and are level 50. Keep repeating this time-changing process to get all the Pokemon you want from him.
I hope I helped a lot!
1. Bulbapedia- Curtis
2. Bulbapedia- Yancy (The Curtis page doesn't have his text listed after the 30th call)
3. Serebii- The Dropped Item Event
4. Youtube- "Pokemon White 2, Part 134: Yancy & the Dropped Item" (also very helpful)