pretty sure he called the friend first so the senpai assumed that he had feelings for him instead and got jealous
No, he broke up with him because Mirai never takes the first step. Mirai couldn't speak up to him, couldn't explain his reasons. If you can't say to your boyfriend that you called your best friend first because you're childhood friends that live together and he would easily know what to do in that situation then you probably have other reason. Is like... If you can't say to your boyfriend where you're going to he would assume that you're doing something wrong, like cheating on him, or that you don't care about him.
Mirai is kinda of stupid and damn lazy...
He broke up with him because he didn't call him? I don't understand that part...
Then who did he call when he got ran over? And who said "Hi how are you?"???