Hello everybody, this is my very first mod, I made it for myself, but decided to share it in case somebody wants to try it too. I'm not experienced in modding at all so please don't judge too hard. What this mod does? Just what it's description says: it brings back in the game some beta phone calls as well as other unused audios along with some minor tweaks. Nothing was cut from the game, I only added extra stuff. Since it involves edited MAIN.SCM, your old save files won't work. Always back up your original files before replacement.

Update:

I released version 2 of the mod. Changes: added beta phone call from Ricardo Diaz; added phone call from unknown person threatening you for BJ's Auto's debts; added beta phone call from Umberto Robina (it does not replace the original, it just follows it); during the mission "Spilling The Beans" you'll hear additional 2 beta phrases from the defenders of the ship.


Flash FM radio now includes the song by Lionel Richie - Running With The Night (as seen in PS2).
During the mission "The Party", Mercedes will comment "I really, like, dig your motorcycle", after her standard phrase "It's so difficult to have a rich and powerful father. Vamos."
Gas grenades are reintroduced in the game.
During the mission "Back Alley Brawl" you'll hear Lance introducing the Pole Position strip club to you.
During the mission "The Four Iron" the two guards at the gate of Leaf Links Golf Club will give you verbal warnings to leave your weapons behind.
During the mission "The Fastest Boat" when you jump in the boat to steal it, you'll hear Jethro and Dwaine comments: "He's taking the boat. Bummer.", "Some square's stealing the boat, man!" 
Added several beta phone calls from Mercedes including the one where she says: "I no longer love you Tommy. I no longer do. Honest.", the one where she complains about Tommy making her hang out with Jezz Torrent: "Never make me hang out with Jezz Torrent again. He is pathetic. Half way through he starts crying about his doggie that died when he was 7 years old and that his mommy never loved him.", and the one where Tommy claims that Diaz died because "there was an unfortunate fire at his mansion."
Beta call from Ken Rosemberg trying to give you a business advice was added.
Unused audio that was supposed to be heard during the mission "Rub Out" is added as a phone call that you'll receive after this mission: "Tommy, man, we did it! We're in charge of this place now; we can take over. You and me. Vance and Vercetti, the two Vs! I like that."
Beta phone call from Phil Cassidy was added (you'll hear it after completion of storyline missions).
Beta phone call from Kent Paul offering you to rob a SWAT's retirement fund was added.
In the Pole Position strip club, you'll get to hear some comments from the stripteaser in a private room, like "You want me, don't you?", "You like me better than the other girls?", "You're big, I like that", etc. Those were in game's files, but unused.
During the missions for Diaz you'll hear the comments of his butler, like: "Tommy Vercetty, Mr Diaz is expecting you" or "Welcome Mr.Vercetti, good to see you again, sir!" 
In Malibu nightclub, you'll hear hilarious comments of the bartender like "Of course we won't serve you" or "You're making me puke looking at you", those were in game's files, but were left unused.


Installation: just copy the files over the original ones in your game directory.