Call forwarding

Your method doesn't QUITE work on my s24, but.... Here is what did work. 

I'm going to forward this identical answer to this SEPARATE question:  "How can I set GOOGLE VOICE to become my VOICEMAIL provider, when I have an ATT account, on a Galaxy s24?"

1. The Simple answer, is to go to GENERAL SETTINGS on the s24 phone, and SEARCH for "CALL FORWARDING"  That brings you to a settings page, that has four conditions that you can set your phone number.   
2. NOTE:  Do NOT try to change the dialer's preset "Voicemail provider", which is set for "your carrier".  (The other numbers on that page I changed, but they did NOT set my GV to voicemail, and I do not know what they do.).
3. First, WRITE DOWN the number that is currently listed for Call Forwarding.  That is your ATT voicemail number that was already assigned by ATT, even if you never "set it up".  They say that your ATT voicemail account will disappear, if you don't set it up within 60 days.  I don't think I care, since I have GV.  But you might want to "set it up", so you could use that number for an EXTRA VOICEMAIL later, for some other purpose in the future, and you'd have an extra VM for a backup service.  For instance, you could tell GV to forward ONLY special or selected calls to that ATT Voicemail system.
4. ADD your GV number  - On the LAST 3 choices, add your Google Voice #.  -- Do NOT add it to the first, unless you don't want to answer your cell number at all, while on your phone.  
5. TEST it: -- I tried calling my regular CELL, and it rang a few times, and then STOPPED ringing the phone, and instantly played my Google Voice recording for callers. 
6. GET your Google Voicemails from your phone, PC, or tablet:  Install the GV APP on your s24, and put an ICON on your "always on, bottom row".  Then you can always bring up the DIALER, and GOOGLE VOICE to see your VM, missed calls, or text messages.  IF you have your phone set properly, you can choose which number AND SERVICE to use for outbound calls, when you hit "call".   I always leave that GV webpage up on my PC, and can manage VM, texts, etc., easier than a tiny phone.   -- Note that most all spam calls will show any Voicemail as "NO Transcript".  It is so frequent, I often test sound by finding a real VM.
7. Optional, but highly recommended:  Forward NOTIFICATIONS in GV to EMAIL and to send a TEXT message to you ATT CELL ##.  That way, you can see what, if anything, they said, and never "listen" to another VM, unless you want to.  -- Oh, and GV is FREE for personal GMAIL users.