This building has 2 phone lines, 555-3334 and 555-3329. We were having some issues with 3334, so we told the phone company to catch any calls for 3334 and just route them to 3329.
When you call 3334, you get this odd issue: call comes in with Caller ID. Answer the call, you immediately get a 2nd call, no caller ID, and it's a dial tone when you grab the call). If you call 3329, this behavior does not happen (phone calls received and complete normally, no 2nd phantom call). 3329 comes in on the PRI.
I don't see this as any type of config issue related to your build. I have to think that this has something to do with the way the Telco is applying this workaround. Have you engaged them with this concern? Have you disconnected all the connections related to 3334?
Just a thought,