Kninjanin, my own solution is not for everyone in this era of total dependence on cell phones, but here is what works for me:
I have one phone only and it is a landline, and the number is unlisted. Plus I almost never give out that phone number. So, in the 17 months I have had this phone I have gotten only one spam call, and I realize from Other Tink's answer that was prolly robo-calling. I answered the phone three times, and each time there was no one there...so I think it was a robo-call and other answering machines picked up before I could get there!