After completing 3 Security Contracts you will unlock the Payphone Hits, and after successfully completing your first one which comes via a random payphone call you can call Franklin to request them at your leisure.
To actually meet the old friend, who is Franklin, you have to buy a certain building from the Dynasty8 Executive. Be careful not to buy the wrong building, because in-game purchases are not refundable. The icon for these properties looks like an upright rectangle with horizontal lines across it.
So to answer that - Franklin is playable at the start, then you unlock Michael after certain missions (robbing his house), and you unlock trevor last, after the first heist.
Once you've bought an Agency, set a waypoint to it using your map or the Interaction Menu and then head inside to meet Franklin Clinton and watch the first cut-scene. Franklin will introduce you to your new business, and tell you to get started on some Contracts while he works on a big client.
Now at the top is where you'll see Franklin the first character you unlock. And the second characterMoreNow at the top is where you'll see Franklin the first character you unlock. And the second character will be Michael. And the third character will be Trevor as you progress through the story.
Navigate to the Old House: Use your map to locate Franklin's old house, which is marked on the map with a small icon. Drive or Walk to the Location: Head to the old house at the address 3671 Whispymound Drive. Enter the House: You can walk into the yard and approach the front door.