iOS is the operating system software installed on your iPhone. To find out which version you have: Tap Settings, Tap General, Tap About, and view the version number.
Your rate plan includes voicemail where callers can leave a voice message when you don't answer a call. You can save the voicemail number in your mobile phone making it easy to call.
When you insert your SIM into your mobile phone, the voicemail number is automatically saved.