No notifications of new voicemails on iPhone despite being able to play them I am not receiving notification of new voicemails on my iPhone 4 (model A1349).  The voicemails are available and I can listen to them if I check but I receive no notification when a new one arrives.

This has been a problem for many months but I've finally missed enough information that I'd like to try to fix it.

I've found suggestions on various websites to use "Reset Network Settings".  I haven't tried this yet because I don't know the full consequences and I'd rather not disrupt the rest of my phone service.
Recently upgraded from iphone 5s to iphone 8 and also changed providers, had 5s call forwarded to new iphone 8 for a month to be certain i would be satisfied with coverage ect. Everything wad 100%, voicemail worked fine. After cancelling old service provider my voicemail stopped showing notifications but were accessable, just didn’t know i had any. After hours of tech support and reading different forums I decided to backup my old phone to my computer and restore to new phone. Now everything is working fine.
Hope it helps someone.