I played through one time just to answer all of his calls and paid attention to the corner all the time and at the end I was sure I answered them all but nope, didn't get the achievement.
Some calls can only be triggered certain ways, like if your slightly late for a story related scoop he'll call you, and if you haven't been to the security room for about 8 hours he'll call you to tell you Jessie is worried about you. So it's one of the hardest to get, and I think the achievement can only trigger if you listen to the full call when he first calls you, I mean you don't get interrupted like hang up or get hit by a zombie.
That's stupid, but thanks for the info.
You know what else bothers me? How EVERY achievement is 20 gamerscore! I fell fifteen feet and got a achievemetn for 20. I beat the game and got 20. Ridculous.
I have to agree there. Zombie Genocider is alot of work and worth at least 50GS for doing it. I hope thats fixed in the sequel.