The silence really amplifies the sound of the vibration, Minwoo thought. Normally if he doesn't have the phone nearby, he won't even notice it. Come to think of it, where is his phone? He threw it somewhere after reading Wonho's letter, trying hard not to try and call his mate again. As Mr. Seo suggested, it is better for him to just visit them on Saturday so they can talk. Since, well, Wonho seems to think he is really mad right now.
It is partially his fault, he did send some messages saying how he is disappointed. That, and getting all panicky without realizing Wonho left him an obvious letter.
Wonho said he was going to be with his parents to think something through. He wrote about wanting to make some plans, and that he needs time. But he also told Minwoo to wait for him, and trust him. And Minwoo did just the opposite and spammed him with messages and missed calls.
