1. Find and download Linphone in AppStore.
2. Launch Linphone and click "Next".
3. Click "Log in on SIP server".
4. Enter the following details:
Username: Your sip-number (for example 111111) from your personal account
Password: Your sip-number password from your personal account
SIP server: sip.zadarma.com
5. Click "Apply". If all settings were entered correctly, the connection status will appear as "Registered".
The current Linphone version for iOS does not work with IP PBX from Zadarma, as the application does not support logins which include a hyphen.
This problem should be fixed by Linphone in updated versions.