If the DID digits don't match the extension then there is only a few ways to get the call to terminate at an extension. An IDC table, which you said there is none. Or a dummy ACD que night call forwarded to the real extension or a phantom that is DCFW to the real extension. So if it is anything other than IDC, which it looks like it is, then if you do a DNB on the actual digits coming from the Telco then it will print out where it is in the switch? So if the digits are 4000 and the extension is 5000. You would do a DNB on 5000 and it will hopefully point you in the right direction. A dummy acd que is nothing more than an ACD que with 1 agent and the NCFW prompt is pointed to the actual DN.