iPhone8 voice mail access

May 13, 2018
I am unable to change my greeting from the default to the custom greeting and also unable to retrieve any of my voicemails. I have visited the AT&T shop and they were unable to fix this problem. I have done all the trouble shooting both AT&T and Apple Inc. has offered. I have a brand new iPhone 8 purchased 5/12/2018 but also had this problem with my iPhone 6.