I want to use a prerecorded message as my voicemail greeting but prevent people from leaving a voicemail

Hi, My iphone has updated and reset my answer machine to default. I want to use a prerecorded message as my greeting and prevent people from leaving a voicemail. I had this set up before the update but can't remember how I set it up.

I don't want to deative the answer machine all together as I need my callers to know they've called me/the correct number but I don't want any voicemails left.

Thanks