Samsung Galaxy S not displaying contacts, only unknown is shown My Galaxy S Duos only shows unknown on the display when I have saved phone book contacts calling me? How and what do I do to get numbers to be displayed?
After some investigation I found a fix that works for me.  Use this at your own risk. 

First, the summary:  The file /data/misc/rild_ril.clip is a single byte,
either ASCII '1' or ASCII '0'.  Somehow this value is being flipped from '1' to '0' during low storage space conditions and causes the problem.  The solution is to either edit the file, or just delete it and let it be rebuilt. 

The steps to fix this problem on your device without losing any data are as follows:

Requirements:
ADB or a terminal/console application.

Steps:


Get to a shell prompt.  If you're using ADB, it's adb shell.  If you're using a terminal/console app, just launch it.
Type rm /data/misc/rild_ril.clip.
Reboot the phone.


That's it. Please let me know if it works or doesn't work for you.