How can I retrieve a deleted voicemail on iPhone?

Open the Phone App

Tap Voicemail

Scan all the way to the bottom of the page and find Deleted Message and tap it

Find the message and tap it

Look for the red trash can with a white line through it and tap it to restore the message

Voicemail is a carrier service

Any voicemails are on their server

On my phone, if I go under voicemail, I see an option to see deleted messages.

If you do not see any way to see deleted messages, you will need to contact your cellular provider