I got a call from this number the caller id said "Boulder Call" I immediately put them on my block list.
I switched to ooma.com and can easily block any number through an online interface. Also, I have a little device called Interceptor that is no longer available but it gives me the ability to block entire area codes. So no 800, 888, etc. get through.
Different Phone companies have different processes. YOU contact your PH company for this information- no one can tell you HOW your ph company handles this
You can also get software - that will do this as well - one posted here - there is also GOOGLE Voice.
Panasonic makes a cordless phone that allows you to block numbers as well. I have one and I set to block all calls that do not have anything in caller id, unknown out of area