Mumble works very well, but: - It's not real p2p, it needs a server somewhere. OP can install one at home or they can use a public one, but it's required (and

Mumble works very well, but: - It's not real p2p, it needs a server somewhere. OP can install one at home or they can use a public one, but it's required (and if you use a public server, you lose some privacy) - Mumble only does Audio, no Video. If you can live with those, mumble is the most straightforward Open Source voip solution to use, and I can't recommend it enough (been using it with my relatives, me on Linux, them on Windows: no problem)