After the 10th call,we can register them in the Xtranceiver ,but don't we have to call them 50 or so times for them to trade with us?Are the locations in which they call us the same places from where we have to call them?
PS:Can I trade with them before beating the Elite 4 if I call them the correct number of times?
You have to go the places where they called you to call them back, but you don't need to stand on the exact square. You have to fly around until you find a location where Curtis/Yancy shows up in your Xtransceiver's call list, since it's random.
The best place to begin is Nimbasa, as you can call them in the city and also the 2 adjacent routes (Route 5 & 16), in addition to Driftveil city & Route 6.
Serebii - Dropped Xtransceiver call locations
This is a list of places from where you can call them.
Oh, and for calling them after they've called you, go to the same place, however it is not restricted to one tile anymore. Of course, they won't always answer, so keep looking around the other areas until they do.
Hope I helped!