[lemcoe9]

This is not completely true. Especially in VoIP, there are usually 1-5 layers of FCC-licensed phone companies involved in the call. The CLEC (Bandwidth, Level 3, AT&T, Comcast, et al) sell their numbers to Class 3 ITSPs like Flowroute, SIP.US, VOIP.ms, SIPSTATION, Twilio, etc, and then frequently that service is once again sold to another vendor that might have an actual end-user using the service. Just because one company's switch says "where the call came from" does not mean anything related to the actual calling party.

[viraptor]

You're completely right. I didn't mean it's going to be a single step. If the answer is: it came from another telco X, you ask them. And repeat.