isn’t actually in the file, you should be able to call youself, assuming that you don’t have a buggy version of X-Lite. You won’t actually be able to talk to yourself, of course.
I cannot think why you would have NAT enabled, but hopefully Asterisk would spot that it wasn’t relevant, when calling yourself.
This is a pattern for the correct line. You may or may not need a correctly completed version to talk to the office system, but like this is does nothing, and, as you don’t have suitable dialplan or sip.conf, you will not be able to make calls to that system.
More generally, you need to do one of:
[ul]- keep re-reading the book and other documentation until you understand it;