Servers used to be closed source, but this is no longer the case.
Proof: https://github.com/wireapp/wire-server

Not subject to US government Gag Orders.

Wire works on OS X, Windows, Linux, iOS, Android, and as a webapp.

In addition to sharing images inline, Wire integrates with SoundCloud and YouTube to embed music and videos.

Wire's fully open-source architecture allows anyone to host a server and selectively federate with the rest of the network (including Wire's official servers). In turn, federation allows for decentralization, guaranteeing stable, robust, and uncompromised service beyond even the lifetime of the company.

Check out their pricing. It cannot be cheaper for secure private communication.

Wire's user interface is quite intuitive to use, still it has everything required for a complete experience. Settings are very clean and allows you to turn on/off some features which might be disturbing for some, like link previews and emoji conversion.