* Recommend Linphone on Android
    (CSipSimple is old and unmaintained, does not support Opus or video,
    and is rumored to hang and drain battery -
    maybe esp. when switching between wifi and cellular).
    * Linphone fails with TLS: Try with Letsencrypt instead of CAcert.org.
    * Setup snippet (when it works):

 1. Install linphone from f-droid.
 2. Select "Use sip account"
    * Enter username, password, "bitbase.dk", your full name (as display name).
    * Leave "Transport" set to the default "TLS".
    * Click "Login"
 3. Select "Settings" from main menu.
    * Select "Network" submenu.
    * Select "Media Encryption" and choose ZRTP.
 4. Optionally: Select "advanced" and "start at boot".