There are a total of 32 lot bags, 32 payphones & 8 invisible walls. Loot bags are brown boxes, these can be collected by walking over them. Payphones are usually found near building and in other areas, press to interact, a phone number will appear on your screen. Invisible walls are fake exists that represent prop walls from T.V shows.
Here is a full location map and details on where to located all Collectibles Guide
Credit goes to Dalez & onestonedjuggalo from PS3Trophies.
*Loot bag 31, 32 and invisible wall 8 can only be found/picked during certain story missions. And should be picked up right away.