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.
RC, my doctor uses a switchboard however he gave a direct number extension for me to use as well as his home number.
My attorney gave me her private number, home number and her private cell number. So it depends on the doctor or lawyer.
I have noted on occasion calls from both offices change in the call number displayed. I normally just let unknown numbers go to voicemail and I return the call. Much easier.
I have noticed so called spammers use a rotation of numbers to try and get around a number block. The phone companies don't care because they make money from the scammers leasing the lines, and make money to have customers block them. It is a win win for the phone companies.