I pay t-mobile $4/month for their advanced spam-call blocking feature, and it works very well. I now get maybe 1 spam call every 2 weeks, compared against my worthless work number that has no such blocking which gets maybe 6 spam calls a day.
It also allows me to block numbers from my sim area code. This is where most spam calls spoof to be coming from. When I recently called T-mobile to ask for them to do this, they said they couldn't help me.
[1] At least in North America, each phone number is assigned a "home switch," which will always be notified where (as in "which phone switch it servicing it" not "at 40°58′35″N 80°8′9″W") it is so calls can be forwarded to the proper switch (think roaming).