It appeared in this chapter
(476p2) and also a few chapters back in
(470p10-11). Baku seems to be discussing some plan with Ryuusei, then Ryuusei goes to ask the developers for the phone. Then Ryuusei mentions later that he called home.
I think the phone is the key part of this, since Baku didn't have any way to communicate outside the island during the match. I have one crazy guess for what this could be hinting - he could've asked Ryuusei to contact Karl to bring an air vehicle to the island, so he could win the silly air-vehicle gamble with Souichi. Though I don't know if the bet is still on since he broke the watch. And as Souichi mentioned, the gamble itself doesn't have much purpose. It would be like Baku to have a double-purpose to it though, like asking Karl to bring something that he needs for the match. But then again, he's not really the type to bring something from outside to win. Also, he could've just done the call himself, no real need to involve Ryuusei, so it seems like there's more to it. Hm...