update 1: https://wiki.mumble.info/wiki/Features It has encryption which cannot be disabled and the advanced audio features make it well suited for reliable and pleasant day to day use. It's low-latency and instead of a centrally managed contact database like most commercial services have Mumble uses certificates for authentication and identification of your friends across servers.
update 2: There are alternative server implementations, one in C and another one in Go, meant to run on OpenWRT in terms of hardware requirements.