All ready mentioned in the transmissionary solution but i found this guide to be ace:http://www.gamefaqs.com/console/xbox360/file/928326/49257
(Credited to CarlottaMonroe)
using this guide i can offer the following further advice:
Best attempted at lvl 50 (or at least with a full inventory space, as you level up pretty fast with all these completed escorts, PP photos etc.)
Dont stress too much about trying to follow all of the timings posted in the guide as they can be quite tight, its often best to save early and plan your routes in accordance with the time available (e.g i found it easier to deal with cliff after meeting kent, then picking up gordon, ronald, heather and pamela along the way back). The more you play you'll get a feel for the games structure and find whats best for you.
The AI can be crazy/unpredictable at times so effecient use of waypoint markers is a must and remember to keep them relatively close to you as you clear a path as sometimes zombies respwan on top of them if you move out of shot, holding hands/ supporting solo surviors through large groups speeds up trips through large packs of zombies as 9/10 times you wont get touched.
The real Mega Blaster and Triple Booked Small Chainsaw are a must, reducing psychos's to a mere inconvienence and freeing up inventory slots to give weapons / foods to surviors. I found a Triple Booked Skateboard usefull as well to zip around the mall (1 Book is required for the Small Chainsaws anyway meaning only two further books are required)
Finally SAVE OFTEN!!!!!!
UPDATE: Reading through some comments it seems that there is a common theme in that one of otis's calls is unanswered. This could be for a couple of reasons - a) there are too many active survivors for a call to come through; there needs to be less than 8 at any one time or b) the player is in the area that the call pertains to.
I found a really good list over at x360a that condenses all the calls and details where you should NOT be at the time (Credit to aPerfektFIaw)
Hope this helps.http://www.xbox360achievements.org/forum/showpost.php?p=9323...