No. When you get a spam call, the caller ID is often not random. It's usually the number of some other person on the spam caller's list of people to call, often someone from your same area code. That way, you see a call from your area code and are more likely to answer it. If you get upset and call it back, you won't be calling the spammer. You'll be calling some other innocent victim of the spammer.
Most of the time the caller id shares the same area code and first 3 digits of the recipient's phone number. Many times the last 4 digits are the same just in a different order.
No, but it could be preventing the equivalent of "here is a random phone number I picked, please 1000 of you dial it".