RedPhone is only for Android although iPhone compatible version exists. But this is not what I was talking about.
The problem is that there is no telephony system that you can use cross-platform, that is open source and the clients are easy to install and use for the average user.
No other heavily-used protocols have this problem, FTP, HTTP, SMPT, DNS, Torrent, Cloud Storage, VPN, SSH all have cross-platform, free and open source clients that are easy to set up and use for the average user. Telephony is the handicapped service on the internet
The main problem, as I see it, is the SIP protocol and the design mistake of relying on IP addresses in the application layer.