I think the players would have to find two way radios in order to talk to each other over distances. In a free for all style of multiplayer, where I assume the players would spawn in different areas, two way radios could be found and set to the "Scan" function to pick up other players broadcasting messages. Batteries could also be utilized in order to keep your radio on and transmitting/receiving. If there's going to be different devices, maybe different batteries? Or is that too far? lol.
With Two Way Radios, two different buttons would have to be set for local (maybe a proximity?) voice communication in game and global voice communication.
Where the player is could also affect the signal of the radio (if they are underground or in an elevator shaft etc).
Now, I understand that rather than using this, most players would just use an outside voice chat program, such as ventrilo or teamspeak. But, players who are wanting a more realistic and challenging experience could use Two Way Radios to communicate and find other survivors.
Edit: Just read Nostramo's suggestions, and I guess two-ways and Cell phones would fall under the same category and perhaps Cell Phones would be better, being more common and more useful for other tasks.
4. Cell phone
Not sure if this is taking it to far, but could be used for a multitude of things from telling time to keeping inventory to allowing you to construct a simple local map to allowing you to contact other survivors. Only downside is that they are bright and use up power.