For those interested, Signal doesn't seem to use ZRTP anymore:
> The new Signal voice and video beta functionality eliminates the need for ZRTP. The "signaling" messages used to set up the voice/video beta calls (offer/answer SDPs, ICE candidates, etc) are transmitted over the normal Signal Protocol messaging channel, which binds the security of the call to that existing secure channel. It is no longer necessary to verify an additional SAS, which simplifies the calling experience.
https://whispersystems.org/blog/signal-video-calls-beta/
And it's not in beta anymore: