That's the art & science of it... you need to social engineer the info out of them. Most of the time, even if you do get a caller ID, it's spoofed, so it doesn't matter what "number" they call you from. You need to identify a business with an address and phone number, and definitively tie the activity back to that business.

Yes, it takes time and skill, but then again, between the TCPA and state statutes you could potentially recover close to $6000 per call. Now when you negotiate a settlement you may not actually see that much, but you are definitely getting the attention of the offenders, and just the process of responding to your complaint (lawyer time) will take another few thousand dollars out of the pockets of these cretin.