From browsing the source and referencing the architecture section of the wiki ( https://github.com/WhisperSystems/RedPhone/wiki/Architecture... ), there is a RedPhone master server (master.whispersystems.org:31337) and a relay server (relay.whispersystems.org:31337) that the phone will use during the course of all phone calls.

Apparently, most cell phone network providers disallow direct peer-to-peer communication, thus the relay server is necessary to complete this kind of encrypted call. All of your encrypted voice data will pass through the relay server, so there isn't going to be much privacy in terms of who you are talking to at what times, but the contents of your voice call won't be revealed.

Most phone networks are behind NAT too, so you need something to relay through.

I would like to see the server software freed as well.

This would also make me feel more comfortable and I think it would also remove some of the risk from whispersys if others could use their own servers. According to the app's privacy policy, all numbers dialed via the app are logged. http://whispersys.com/privacy.html