my voicemail keeps saying that it is unavailable even after i reset network settings and phone why?

I have a red dot that appears on the call log icon and when i click on the icon it has to do with the voicemail but when i try to set it again nothing happens i just recieve a message saying that it was unable to fecth greeting and is followed by another message saying that the voicemail is unavailable to try again later and ive tried numerous times for the past month or so if not longer and nothing changes even after reseting and going to my service providers store to check if it was anything to do with my plan or anything of that nature but it has nothing to do with it, What could it be and is there any way to fic this problem without loosing all my stuff on the phone and with out having to spend money or at least a lot of money.😕😠