I'm guessing you have an Android phone.
It's some Google service that is identifying them as Spam likely.
It may not do much good, as the Spammers just pick caller IDs, more or less at random. You can report the call to donotcall.gov but it won't do any good because the ID was fake. One favorite trick the Spammers use is to pick a random number with the same Co (3-digit) prefix as your phone. This way the fake call looks like it's someone local. I have my phone set to reject any call from my phone's Co, since I don't know anyone with a number close to mine.
If I don't recognise the number I "reject" the call, and promptly "block" the number. Since 2004 I have had the same number, and the numbers after my area code are AT&T numbers.