Hmmm.. I just got hit with a similar problem a couple of weeks ago. Pretty much any call that comes from anybody that's not in my Contacts comes across as UNKNOWN.
At first I thought I was just getting a lot of telemarketer calls and was ignoring them. But then I realized they were actual calls - calls from clients, business and social contacts, pharmacies, doctor's offices, the kid's school - etc - all of them showing up as UNKNOWN.
I called AT&T who had me reset a few settings on their phones and some things on their end. They couldn't resolve it. They escalated it to second level support, they couldn't fix it either.
They escalated it to third tier support who claimed they fixed it. But the problem kept occuring. Then the guy asked me to list all the people who had contacted me because their phone exchanges must be 'out of compliance.' WTH?!?!
Caller ID has worked just fine the 12 years I've been with AT&T, I have a hard time believing that somehow standards have changed. This is getting more than a little frustrating.