Latest versions of it all
It's of paramount importance to update immediately not only FreeSWITCH, but all your software and your devices' firmware to the latest versions, as soon as they are released.
Specifically, pay attention to the new releases of phones' firmware; they close security bugs and add security features. When a new version of a software or firmware is released, the security bugs that are fixed become the "features" that attackers are looking for in systems that have not been updated.