We have been extremely happy with Mantis as a self-hosted approach. It is easy to use, yet has the sophistication and flexibility for a needed to manage a project of non-trivial size.
On the other hand, Digium recently moved away from it in favour of JIRA for issues.asterisk.org.
For fairly large projects like this one[1], I have always favoured internal hosting of such systems in order to maintain maximum control, use optional plugins, make customisations, etc. I think that would make the most sense for the SR/Kamailio community.
-- Alex
[1] It's not nearly as large as say, the Linux kernel, but it's bigger than 99% of open-source which, after all, consists largely of projects done by one person or a few people at most.