Not just 800 numbers Caller ID data ALWAYS travels with a phone call. If the caller has his number blocked, that merely sets a flag, which it is up to the receiving carrier to respect. Most do, but to varying extents. One of my cellphones (the hobby one, of course) is a prepaid model - I will not mention the carrier. When a call is received from a blocked number, the calling number is not displayed in real time. BUT if I go to the carrier's website an hour or two later, log in and check my usage - the blocked number WILL be shown! I will admit that I have collected one or two numbers this way! So ya never really know how blocked is blocked . . .