voicemail cannot be setup:-

I don't usually do things with "plain vanilla" Land Line service, but I think what they are telling you is that your phone is not sending Touch Tones to their system.

This could be due to your phone sending a SHORTER touch tone signal to their system than they are set to receive.

I suggest that you look at the manual for your telephone and see if there is an option to change that setting. (Touch Tone duration)