Guys, calm down. There hasn't been a point so far in the manga where Kazuya and Sumi have exchanged contact information. In fact, according to industry standards, rental girlfriends giving out their private contact information to clients is grounds for getting fired. Even Chizuru was only able to justify doing so first because her friends had added her into a group message that Kazuya happened to be in and afterwards because he became a producer for her movie. Sumi has not had such opportunities to give her info to Kazuya so Kazuya cannot contact her except through either the company as a client or through Mizuhara (which, for obvious reasons, he's avoiding doing). Now that he's contacted her and got her there though, she's free to spend as much of her private time with him as she wants.