I agree with what has been said previously. There isn't a way to detect a spam call before it actually happens, especially with the way most telemarketers or spam callers can alter the number that they call from. It is your job as the user of the service to not answer calls that you don't recognize. Just like Verizon cannot be held responsible for the calls you make they also cannot be held responsible for those that you receive either.
FamilyBase is a service that I use and for the $5.00 per month I get a lot more out of it than just blocking unknown or restricted calls, so I can understand how you wouldn't want to pay that just for that feature. Do you have a smartphone? I have one and use an app to block unwanted calls, including those from blocked or restricted numbers, There are so many of them out there and they may solve your problem.