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