This trophy is missable. I have written at which point you can no longer get it in the spoiler tag, but if you don't want to read that then just get it as soon as you get the cell phone.
At the start of the game, after Strong Bad checks his e-mails press Triangle to open the map, then press X to draw The Track anywhere on the map and press X on it again to go to it. Once you get there speak to Homestar (the tall guy with a red shirt and no arms). After he speaks to you he will throw his cell phone away, go pick it up. Open your inventory with Square, choose the cell phone with X and them press L1 or R1 to cycle through all the stuff you can use it with until you can choose Strong Bad, press X on him. Pictures of all the different characters that you can call should show up. Choose them all and you should get the trophy.
*Note: You must be in a different area than the character you are trying to call, e.g. you can't call Coach Z from the track.
After you have called everyone you will get the option to call Homestar. The trophy will unlock after you call him.
Characters you need to call:
After you call each person their picture will disappear, except fir Marzipan and Homestar. You only need to call each person once.
Please vote positively if this guide was helpful and feel free to leave a comment if I can improve it in any way.