I am trying to set up my voicemail but it will not let me use the letters, only the number come up

I don't understand what you are setting up using numbers - or letters. If it's a passcode, then you can translate the letters into numbers as you would on a telephone keyboard - for example, an "S" would be a 7.

But that may not matter, as the process for setting up voicemail is strongly tied to your cellular carrier. If the above doesn't answer your issue, contact your cellular carrier for setup assistance.