* It is free software, or rather, the client they recommend is Linphone, an existing open-source VoIP client.
* Specifically, the value add is an introduction/routing layer over SIP. They recommend connecting via Tor.
* The encryption is stock ZRTP.
In comparison, Signal/TS is free software, but uses novel crypto for text messages. I believe RedPhone/Signal voice is still just ZRTP. RedPhone/Signal will convert the SAS code to two frequently-amusing phrases, whereas LinPhone will just display the raw code.
There doesn't seem to be an easy way to use RedPhone with Tor, or to anonymously register with RedPhone, though I could be wrong.
Redphone/Signal has its own signaling protocol for voice calls as well. This service uses SIP. The Redphone protocol is simple in design, while SIP is the opposite.
In comparison, Signal/TS is free software, but uses novel crypto for text messages. I believe RedPhone/Signal voice is still just ZRTP. RedPhone/Signal will convert the SAS code to two frequently-amusing phrases, whereas LinPhone will just display the raw code.
There doesn't seem to be an easy way to use RedPhone with Tor, or to anonymously register with RedPhone, though I could be wrong.