Here is the easiest way to see what is happening with the number, call it from the intercom. What do you get, if you get a warble tone, the number you dialed does not exist in your system. If you get a busy or ring, the number exists somewhere. It is either most likely an ext or adjunct. If you want to go the dective way about things, go to sysrenumber-->extensions, type in the number in question and if you get the beep, it is not an extension. Then go to adjuncts, any beep??? Yet, like Rickbro said, the simple way to go about things is do a print out of your dial plan. That will tell all.
As for the set-up space, the minute you go to Default numbering-->set-up space, that is it, the first port on you system is now 7100. The only little bit of warning you get is the system has to be idle to do it. You will have to redo the dial plan in you switch, all other translations stay the same.
Good luck