Yeah, I don't even know how that would be possible unless it's owned by one of the phone companies and is only available for their users. I highly doubt any telco would share that data publicly, especially if all you need is the subscriber's number.
This is likely done with something called an HLR lookup via SS7. I believe it was explained to me that HLR lookups are meant for call setup and SMS routing, NOT for exposing network information via web services, but that doesn't stop companies with SS7 access from doing it anyway since there are plenty of people willing to pay for that information.
