Once you have taken calls from Yancy/Curtis 10 times, they will arrange to meet you in front of the Rondez-View Ferris Wheel, which is located at the far left side on Nimbasa City. There, you will give Yancy/Curtis back their Xtransceiver and you will register each other in your own Xransceiver. After that, Yancy/Curtis will appear in your Xtransceiver to take calls from you at random times, so long as you are at one of these locations.