I have Verizon, forget Verizon's block numbers. Check out PrivacyStar. I have had it for 4 years and it is the best. Service is free unless you want the look up feature, then it is $19.95 a year no contacts. No limits to block numbers Phne never rings from known telemarketers.
The number is generated from the internet. They are calling from a computer. This is why when you try to return. The number it does not work. It is a way for them to call, sometimes harassing, without being identified. I have learned now to just not answer unknown numbers. Thank god I am not in a career yet where I would possibly have to pick up unknown number calls for work. This would be maddening.