Yes it warning that is an Unencrypted call because it is a GSM call which will never safe, so Purism is implement Encrypted calls on Phosh under the core app of Calls plugged with Chatty by using Matrix and XMPP, So when Purism finish all this developments then you can do Calls Encrypted on the app Posh-Calls. There is a free software XMPP powered app which would be the first and soonest to make an encrypted call under Libhandy-Phosh. Stay tuned