I am not a CentOS expert, but the repository structure seems a bit strange. E.g. for CentOS 6: [link](http://rpm.kamailio.org/centos/kamailio.repo) - this contains repositories for 5.2 and 5.3. branch. The 5.2. and 5.3 branches contains old versions, like 5.2.5 [link](http://rpm.kamailio.org/centos/6/5.2/5.2/x86_64/Packages/k/). The newer versions are in another location, like this one [link](http://rpm.kamailio.org/centos/6/5.2/5.2.7/) but does not seems to be found if added in the system.