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!