In many cases this is controlled by your carrier... We are also a carrier and our default is to send the main billing TN number as Caller ID for a call if no CID information is sent with a call from the phone system. Some carriers won't even allow a call to complete if you are not sending a valid Caller ID with an outbound call.
If they want this for all calls, I would suggest asking their carrier if they can do this on their PRI. I did see one customer one time requested a certain DID have the CID name listed as "ANONYMOUS" and then on their phone system they pointed the DID to a recording that said something like "The number have dialed cannot receive calls" or something like that. Be aware doing something like this could have E911 implications, as by most state laws a number sent to E911 must be able to receive a return call so you have to setup CESID properly.
