You can use the Auto Attendant to block these calls. The AA has 3 options (ring the phone, make a new call and callback). These options can't be changed. You need to change the initial prompt to something like "Enter 1 to complete your call" then delete all the other prompts. You also have to make changes to have options 2 and 3 fail if accidentally selected.
This can be setup only on SP2.
The AA is answering the call, so you will be charged.
You can define a white list of known numbers to bypass this procedure.
NoMoRobo is a free service that will block some robo calls. Some robo callers use spoofed callerids. These can't be blocked by NoMoRobo. You can't use the AA and NoMoRobo together.
https://www.nomorobo.com/ Edit:
NoMoRobo will also answer the call, so you will be charged. The advantage is people don't have to listen to a prompt and enter 1. The disadvantage is some robo calls will get thru.