I’ve had an issue where missed call and voicemail notifications are delayed for several hours or even a day or more on my iPhone. I switched to T-Mobile recently and can’t remember if I had this problem when I was a Verizon customer, but regardless it has become annoying for me and friends/family who leave messages that I don’t receive.
Others have had this issue too, and one T-Mobile customer suggested that having too many deleted voicemails caused the problem. I am pretty good about deleting voicemails once I listen to them, but I did not realize that all of my deleted voicemails were still available in the Deleted Messages section. Mine went back almost four years! I guess kudos to Verizon/T-Mobile for transferring them across services and never permanently deleting them? I assumed deleted messages would eventually go away after a certain amount of time.
I had a few deleted messages that I wanted to save, but I manually deleted several hundred voicemails from the Deleted Messages section. After that I started getting missed call and voicemail notifications right away again.