Cent OS 5.XX below

Asterisk Asterisk Support
3

  1. Centos 5 EOL March 31st, 2017, so you should be thinking upgrade your system or you could be hacked

  2. Elastix as you know it is dead, the company has been sold to 3CX and not using Asterisk Framewrok anymore, So you can try FreePBX or maybe some Elastix folk

  3. According to this thread Asterisk runs on Linux.the Developers team test on CentOS 6 and Ubuntu 14.04. Anything beyond that is up to you to discover.