Drake sings this when talking about his phone ringing. "When that hotline bling" just means "When that phone rings" in a black-culture way. He sings about how he knows when his phone rings late at night and the caller is the
particular female character from the song, that it can only mean it's a
booty call.
Contrary to the other ultra-white
definition on here, "Hotline bling" is NOT just some term for "booty call", not even close. It's only relevant in this one song.
Dude 1: Damn man your hotline hasn't stopped blingin since we left!
Dude 2:
Yeah dude it's the bossman, he keeps asking me to work this
weekend. Only he can make dat hotline bling
like that... I ain't takin no calls!