First, in GTA 5 take out the phone.
Then dial this number: 1-999-759-2837.
Wait until it dials.
When it did, you'll hear a specific music.
As ChatGPT says:
"The number 1-999-759-2837 (1-999-SKY-DIAL) in GTA 5 is a special Easter egg that, when called, plays a music. The music you hear when calling this number is part of the fun and quirky hidden features that enhance the game’s experience."