Asterisk 13 has no more or less problems with nat than asterisk 1.4. So you're likely headed down a blind tunnel and going to find yourself right back where you started. Just sayin' ...
These are not my examples to "share", they are already visible on this forum and others. But i don't recommend it. If you are prepared to reinstall, and want 1.8, just reinstall with Vicibox 7.0.4 or 6.0.4 instead. Those .iso installers (and others) are available at:
http://download.vicidial.com/iso/vicibo ... r/archive/Note that any version of the Vicibox installer will still install the Latest version of Vicidial. But older versions will have an older OS and other packages, including asterisk. The installer's job is ONLY to provide the "environment" for Vicidial to run. So old installers still work just fine. That's also why upgrades continue to work even if the Asterisk version is old.
Upgrading a Vicidial install does not involve changing the asterisk version. So if someone installed Vicibox 6.0.4 months ago, and they upgrade, the Asterisk version will remain unaffected. All that gets upgraded is the Vicidial Scripts and the DB structure.