It sounds like your registering via SIP just fine, but your new "n800" account isn't included in any of the incoming call rules in your extensions.conf file. I'd take a look in there first.

Don't worry about the 405 "Method Not Allowed" messages, I think those have to do with the mailbox=2100@default line. As far as I can tell the N800 Rtcom stuff doesn't support voicemail info (#messages waiting, new message, etc).