Blocked CID information is still transmitted between carriers, it is just flagged for privacy so that carriers will not transmit the information to end-users. Enterprise voice connections get all of the data (ie, if you think you're calling a Fortune 500 company anonymously with *67, think again) and CLECs can sometimes provide the data in the SIP header on enterprise or carrier-class trunks. I suspect VOIPo's upstream carrier is transmitting this information to them, and certain elements of their infrastructure are using the CID info.

This "feature" was made semi-famous in 2008: http://news.cnet.com/8301-13554_3-9997976-33.html

(note: FlowRoute closed their infrastructure to providing the full call details on blocked calls shortly after this went public)