Linphone

Install, create account and configure Linphone.

Account

Create account

1)....

Recover account

Open https://www.linphone.org/free-sip-service.html&action=recover on your browser.
Username is: "YourName" without @something.org and email you used to create the account.

Linphone for Android

Install Linphone

on Android...

Configure Account

Select "Use Linphone Account"
Check box "Use your username and password instead of your phone number"
Username is: "YourName" without @something.org and password of your account. Domain is; "sip.linphone.org", check box "TLS".

Activate Encryption

Go to app menu and select settings.

Linphone for GNU/Linux

Install Linphone on GNU/Linux...