Maybe it's running a buggy release that leaks memory so the system is swapping all the time, maybe the server is simply hacked and is busy mining some crypto, maybe your VM doesn't have efficient paravirtualized drivers and uses slow emulated devices. There can be a 100 plausible causes, it's waste of time trying to guess what might go wrong. 50 concurrent calls should not be a problem at all even with 1 CPU. Try using top/htop/atop/iotop to find the bottleneck.
Hello, after I deactivated the g729 codec has improved, I'm analyzing and thinking about moving from virtualization host, go for a XEN or VMware.
Thanks for the feedback.