prindle@NADC.ARPA (01/16/86)
You can determine which carrier you actually get by dialing 10xxx-1-700-555-4141 and listening to the recording you get. This may not be foolproof, as the CENTREX may handle the 700 differently than calls to D.C. from Philadelphia, but, more likely, it just gives up trying to route the call when it gets the 10xxx.