Select an option from Settings > Sounds & Haptics > Ringtone to set a new default ringtone. You can use a third-party app to create a custom ringtone. Select it as the default in the Ringtone settings ...

You can set up voicemail on your iPhone through the Phone app, which will let you record a custom greeting that you can edit at any time.

Here's how to change your iPhone's default notification tone.