Well, because of this question and because I am in the middle of a W2 playthrough, I decided to go get all the calls I needed to access Yancy at the ferris wheel in Nimbasa City.
I can confirm that you need to be called by Yancy/Curtis ten times before they will offer to meet you. Every call has a different dialogue with Y/C, regardless of where you are.
I am a little more clueless on the second part. After you have a talk with y/c, you, the player character, gives y/c the dropped item back. Then you add them to YOUR xtransceiver. Then, you are able to call them from any of the marked Y/c spots, (or the ones they were able to call you from originally.) You can only do this once however, and I am kind of confused on how to reach them again, as I have never done this before in B2W2.
Idk if this helped at all fizz but I just wanted to put a few facts on the table