OpenVZ is useless for Asterisk or any other resource intensive application. OpenVZ was built from a hosting provider point if view, and if you exceed any of the counters, dozens of them, they system will kill your app immediately. It is almost impossible to build a VPS that will use all the resources of the machine. The only container technology that works for Asterisk is LXC, better implemented by Ubuntu on the server side. CentOS 7 is behind in the LXC version and it is part of the core OS, but found in a repository. Yo need kernel 4.X to make it works flawlessly.