He may have taken a sleeping pill. In any case, he didn't really want to talk to her and they were broken up. I'm sure part of why he kept talking to her was because he knew there would be huge drama if he didn't. It's easier just to let them blabber on (I say that from experience). It might sound mean, but relationships with disturbed people can be very tricky
Of course it's better to get 100% away and have no more talking, but that can make the person go bonkers and cause a lot of problems. Certainly huge drama
I think he was confused how to handle that phone call at first, for good reason, probably. If he sounded too attentive and polite or well-spoken, that would encourage her worse than sending obvious signs like he did. She didn't pick up on the signs that he wasn't interested, but they were there.
And he sounded very sleepy
I'm not discounting those who think he was just tired; I just think he was also sending signals on purpose.