I have setup a handful of UCM 6204s and have not had this issue. Using Cox phone services and GXP2135s. When I dial out to a long distance # or 800# I get either a notice saying number is incorrect, or when using a remote handset, nothing at all.
I have my national outbound rule setup and using _XXXXXXXXXX as the pattern. I am not sure if I am missing something if cox requires an append to the number or something. Anyone have similar issues? The phones work for local number, and non local numbers with the same area code.