As containers use LXC (a form of "jail" or "openvz like") to run a program on the only kernel that is running on the actual hardware ( instead of mimicking a machine and running a kernel ), FreeSWITCH will probably behave well, if it is the only program running on the actual hardware.
If on the same hardware other programs (containers) are running, you will have exactly the same drawback than running FS on an hardware machine that is running other programs too: contentions of resources, IRQs, timers, kernel structures, etc (eg, a no-no, under high loads).
So, containers are better than virtualization (as in KVM, VMWare, etc), but nothing will beat FS running alone in an actual hardware (containerized or not) for obvious reasons.