"There still exists some uncertainty with certain phone calls, but for the most part, everything is now crystal clear with how the game "works"" - Kyle's comment on his Classic% PB

Is there any place I can see exactly how phonecalls "work" and the decision making behind the current IV Classic% (same as Any%?) route?

Rampage start either Trial by Fire or Uzi Money. When you complete the objective, the mission and the rampage will both pass (as long as the rampage didn't fail before the mission objective timer starts). The counter for the rampages doesn't increase, but completing all other rampages gives you the percentage (with a 18 or 19 out of 20 counter).

In addition to this, it can be used along a remote item pickup to skip the drive to and from the flamethrower in Trial by Fire, as can be seen in

Note: If the mission fails, the rampage also fails. Note: Simply picking up the flamethrower remotely doesn't spawn any triads in Hepburn Heights. The Uzi rampage is spawning the triads.

Starting a sub-mission (Taxi, Firefighter, Paramedic and Vigilante) right after savewarping seems to not change the on_mission flag value. This allows savewarping in those missions, similar to the method described in http://www.speedrun.com/gtavc/thread/z83eh, but involving no rampages. The timing is precise from what I've tested but you can try again right away. Practical applications of this glitch would be to use savewarps during Taxi (not Paramedic, as health does not get replenished after every level), or, if it works without crashing, duping Taxi fares, as seen in the video below.

edit: Duping taxi fares work. I saved after 98 fares (not sure if the save is needed but I did it for safety) and did the last two normally. Took just 67 minutes, but could easily be 55 or less with good execution and RNG.

edit:

Not sure if this is known, and it isn't really useful as far as I can tell, but it might inspire some ideas.

By starting a Grenade Rampage and exploding a few vehicles, they can explode again during a replay (as long as the replay is long enough, in the video I fail to get a long enough replay twice). If the rampage ends during the replay, the game gives you (30000 - the number of grenades you used) grenades

After throwing the grenade, run between the tree and the wall at a specific angle to clip through the wall. After the kill, grab the Quad bike, or buy the Quad if it got destroyed.