The feature is not in 4.3.x, you can't enable it there based on my check to source
code -- it was added after the release of 4.3.0. It might work by just getting the usrloc,
registrar and nathelper modules from master branch and overwrite those in 4.3.x branch.
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https://github.com/kamailio/kamailio/issues/310#issuecomment-139501542