Setting up voicemail

Sooo I’m trying to set up my voicemail & all the research I’ve done says something completely different than what I have on my screen. It says to go to the phone app on the Home Screen, did that. Then it says to go to the voicemail tab on the bottom right, did that. But then it says to press “set up voicemail”, I don’t have that. I have “call voicemail” and when I call it says “to leave a message enter the 10 digit mailbox number of the person you are trying to call if you have a mailbox on this system please press pound” so I put my phone number in and it says “sorry could not find a mailbox for that number” like nooo duhhh I’m trying to set it upppppp. Someone help me please, what do I doooo ? My phone is atat provided and I’m struggling !!