You are subscribing, thus get a bill. They do not subscribe to your carrier. They call in from a peer, often VOIP such as Magic Jack with invalid location data.
VOIP service other than consumer packaged plans permit users to set their own CID info. For example, I can enter my 800 number for call backs.
Commercial lines are different from home lines where the in and out line is one and the same.
I have a DID Direct Inward Dial number. It can't call out.
I also have several trunk lines. They can call out, but can't take a call in as they have no number. On those lines, I can place my own CID info per call, or by extension as needed or for abuse.
Ever dial 9 to get an outside line? Several hundred phones may be using 3 or 4 outgoing trunk lines. CID can be set for the hotel 800 number or each extension can provide their own CID so return calls go to your shop 800 number or cell phone.
This feature makes fake CID child's play.