How do I set up the phone message for missed calls?

Hi there Tadams6,

We understand you're looking for information on options regarding calls you're unable to answer.

On iPhone when using a supported carrier, you're missed calls will go to voicemail, which you can view and manage through Visual Voicemail.

The following page offers information we think you'll find helpful: Set up Visual Voicemail on your iPhone

Cheers!