I'm not sure if this was suggested before, but I think VoIP can help immersion a lot more with a few (and maybe not so easy to implement) tweaks:
Attract nearby zombies by talking.
It would add a new level of danger, and force players to be aware even of when they communicate, making looting more exciting. The radius of the sound could be related to the input volume, so that the player could shout, whisper or talk normally, with different results on the environment.
Add radio effects to the voice
This is purely for immersion and not so much for practical purposes, but I think being able to hear through a voice radio effect can help a lot in this department. Maybe add static depending on the distance from the other person, the quality of the device or even the battery remaining.
As I said, a couple of changes, I'm not sure how hard to implement or if this was discussed before, but these may be interesting changes to improve immersion in MP games.