It’s because the ml has really bad headaches and mental issues. He calls cocaine so he can heal him through his singing. As for the sudden shift of interest from cocaine to yaba, from what I understand, the ml’s interest in cocaine isn’t romantic, it’s just because of his healing power. But the ml’s interest in yaba is much deeper, it’s romantic and possessive
I checked the novel chapter again (it’s been a while since I read it, so I forgot some details ). From the novel, before the ml calls Cocaine, he actually takes cocaine powder to relieve his headache after his mother calls him. But it doesn’t heal him fast enough. He needs a stronger hallucinogen, even if it’s cheap and leaves a dirty aftertaste. So, he ends up calling Cocaine.
When Cocaine asks why he called at that hour, the ml is literally speechless. At that moment, he suddenly recalls the “cheap drug’s song” he tasted a while ago, which still leaves a lingering aftertaste in his mouth. And do you know what that “cheap drug’s song” means? It’s Yaba Yaba (in Thai language) refers to a cheap but strong drug often called the “mad drug” or “crazy drug”.
So basically, that question reminds the ml of Yaba’s singing and he unknowingly craving it. For now, he’s still in denial or not fully aware of his feelings for Yaba.
I’m actually not good in giving explanation ☹, so here’s the lines from the novel:
(Ch. 11.2)
Cha Yiseok sucked in some more powder. As time passed, the ecstasy did not come easily.
Suddenly, he craved a strong hallucinogen. A high-concentration hallucinogen that would unleash his instincts for a moment and make
his mind chaotic, even if it was a cheap drug with a dirty aftertaste that would stop this headache now. Cha Yiseok picked up his cell phone. He moved his thumb up and down to scroll through the phone book. After a few rings,
"Hello..."
A clear boyish tone came over. Cha Yiseok stretched out on the office chair with
unfocused eyes.
"Have you been thinking about me a lot?"
A faint laugh tickled Cha Yiseok's ear. Then he heard a calm, refined voice.
What brings you here at this hour..."
Cha Yiseok was uncharacteristically speechless. Because the intense grains of the cheap drug's song remained in every corner of his mouth. The aftertaste did not disappear no matter how much he washed it away and made his mouth burn. It was because he hadn't heard Cocaine's song for several days. There was no other reason for this thirst. He tilted
his head back and closed his eyes.
"Sing me a song'"
Yes, it was simply the effect of the drug

the story is very intriguing but confusing at the same time. I read the novel spoilers before continuing but.. why did the ml call coca just to hear his voice? The part where coca had to go to the bathroom? And why the sudden shift of interest from coca to yaba (or is it not sudden at all?) everything's so blurry for me to understand haha. If you guys can provide more details that would be great!