From Guardian Project Wiki
Groundwire is a proprietary iPhone application with an imposing price tag to bring it up to spec for OSTN. It's available through the Apple app store for US$9.99 as crippleware, which disables three things, NAT detection, g729 audio codec and outgoing ZRTP. For an additional US$24.99 "in-app purchase" you can repair the outgoing ZRTP call feature so a single user may make a secure outgoing call.
Groundwire was registered successfully with the ostel.me test bed. It was able to make an outgoing call. The hardware was an original iPhone with firmware 3.1.3
Install Groundwire Go to the App Store and search for Groundwire - Business Caliber SIP Phone under iPhone apps. It currently costs $9.99. Click install.
Now that Groundwire is installed, go to the applications folder and open Groundwire.
Groundwire Wizard Settings Opening Groundwire for the first time should bring up the "Add Account" menu. At the top of the list, when sorted by popularity, there should be an option for "Generic SIP account". Click this.
Enter the following SIP account details: Title: OStel Username: [user we sent you] Password: [pw we sent you] Domain: ostel.me
Under Advanced Settings please change the following:
Transport Protocol: tls (sip)
Click Done once those settings are changed. You should see it read "Registering" in the top left of the app for a few seconds. It should then turn into a Green phone icon with arrows pointing towards and away from it.
Congratulations! You've successfully signed in.
Note: For outgoing calls ZRTP encryption, you must buy the add-on. This app does accept incoming ZRTP connections.