I have never personally used the service, but the design from the ground up on ghost call is encryption, using all open source phones/etc (I think ostel does this as well) I also wanted to prevent any unencrypted client from connecting either intentionally or by misconfiguration.
So the project was built for a hobby, I wanted an encrypted phone service. But I wanted every aspect of it to be encrypted, from the signaling to the RTP. I wanted to make sure that no unencrypted client could connect to the platform. I would be interested in peering with oslec though.