When you reset network settings, all of your iPhone’s Wi-Fi and cellular network settings will be erased. This includes your Virtual Private Network (VPN) settings, your Bluetooth devices, and your Wi-Fi networks — so make sure you write down your passwords first! We do this step because it can be very difficult to track down the exact source of a software problem, so we reset all network settings.
If you don’t make any changes the default iPhone voicemail message is pretty bland and not very professional. It is a good idea to change the iPhone voicemail message to something more professional or at least something that lets callers know they reached the right person.

The much safer option is to do this with a third-party voice mail system like Google Voice (but the voicemail recordings are limited to 3 minutes). So it's better to have a paid voicemail service, use the Google Voice recording option for incoming calls outlined above, or spring for the paid recorder services like Recordator.
By following this walkthrough, you’ll have your voicemail set up in no time. All you’ll need is your iPhone and a few spare minutes. Begin by tapping the Phone icon that is located on the home screen. Next, you’re going to touch the Voicemail tab that is located at the bottom right of your iPhone screen.