You can eliminate
/system from your list: It is mounted read-only, and (usually) never changes (except when flashin a ROM or applying an OTA).
We can also eliminate
/sdcard as possible candidate, for multiple reasons: First, it still has 2 GB free space available. Second, Android devices should also work without an SD card inserted. And third, plus most directly clear: The error message says "Phone storage", not "External storage" or simply "Storage".
Which leaves us with the other two candidates (if your above list is complete).
Usually, this alert refers to
/data/data, if that's a separate partition). Reason:
/data is where your apps store their data, and also where apps are installed -- so here the activities take place: Installing and using apps. Don't be confused by the "relative value" (about 30% free space). It's rather the absolute value which counts (here: 1,313 MB). Considering that some devices ship with less than this available altogether (e.g. HTC Buzz), this should be sufficient free space, and thus not trigger such an alert.
Which would leave us with
/cache as the most likely candidate here.
You can ensure this by deleting some caches (you can do so from Applications → Manage Applications and walking through the apps, hitting "clear cache" for each one that has caches -- or use some helper app which does this for you, like e.g. App Cache Cleaner or Easy Cache Cleaner. If this makes the error disappear, it was
/cache triggering it -- otherwise it could only be