You can set an emergency contact via a contact card or using the Health app on your iPhone. To do this using the contact card ...

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 ...