Nice find with the car radio! The way I was doing it is after 2 rings if they don't answer you will get a third ring then the answer phone every time, but your way is quicker.
It goes on the time of day I am sure. When switching characters time fast forwards. I usually would always call people after each mission or try randomly whenever through the storyline. Best way to know if they going to answer-
Hey I don't know why, but I've noticed some of these optional phone calls don't always work. For instance, the one with Dave when you first gain control of MIchael worked for me fine during my first playthrough, but on my 2nd and 3rd playthroughs he didn't answer, but he did during my current 4th playthrough.
Also I was only able to get both phone calls with Amanda after Marriage Counseling during my first playthrough. Not since. And I also remember you could call Tracey a few times while Michael was in Sandy Shores and she would scream at Micheal and hang up the phone. All I got was her answer phone during my current playthrough.
It's only just really dawned on me, though I've known since day one, that the missions have alternative dialog when you retry a failed mission just like in GTA 4, but I guess I haven't failed many missions because I've only heard a couple of these. I think during my next playthrough the campaign I'm going to fail most missions at the start just to hear the different dialog.
Get in the car, turn radio on and call who you want. The music in the car will lower it's volume and so you know whoever you are dialling is going to answer your call because car radio volume automatically lowers.
Just finished The Third Way, this is probably known:
Trevor can call Lemar. He will thank him for his help, but as Lemar says, "Nothing says thank you like a gold bar, home!"
Can also call, Dave. When Michael is finished smashing that car dealer (forgot the characters name) you walk out (or steal Jimmy's car and drive out if you want to keep it for the number plate) and call agent Dave.
He will answer and ask about your family.
Trevor can also call the Chinese gangster (Chang. SR.) at some point when Michael is kidnapped and make fun of the Chang. SR. - this might also be only be a short time AFTER Franklin saves Michael. I can't remember but Chang. SR. IS PISSED off and Trevor gives him some smartass responses as Trevor would, haha.
Can call most people who involved or minor characters in the mission or storyline progress after each mission.
Like, Michael calling Devin up after Molly is killed trying to explain.
Michael calling Solomon up after most of the studio missions and when he (Mike) is hiding in the desert.
When Michael is in the desert if you play as Franklin you will get a call from Amanda asking about Michael.
I'm sure Michael can try call Martin when in the desert or it might be Martin calls Michael and abuses him? Just try call most people after the story mode mission.
Franklin calling Devin when Molly says the cars payments are invested from the cars boosted. I called everyone and some say "call" some just you try on your own terms. Can call people after you hangout and have dropped them off and they answer but will say some rejection line to not hangout again in that same day/night.