Wire works on every platform (you don't need a cellphone to sign up, only e-mail), and they released their server code a while ago as well, and documentations on how to set it up is work in progress, which means that in the future (actually even now, but most people don't know how to set it up :P) you won't even have to rely on their servers, you can easily self-host one.
https://github.com/wireapp/wire-desktop