I have an iPhone 8.
For the last 6 months or so I've been getting missed calls with no voicemails. Then every 3 or so weeks I'll get another missed call and it'll leave a voicemail. But instead of a little "1", it will be a "3" or a "4" and it'll turn out that I have 3 other voicemails from the missed calls I've been getting for the past few weeks.
I haven't been able to figure out what magically bumps those unknown-about voicemails to the notifications stage, it's not like it's always the 3rd voicemails that kicks them all to my notifications.
I have called *86 and there are no options regarding this.
My "deleted messages" section is cleared.
I'm extremely frustrated and can find no answer for this. I just found a voicemail about a $100 rebate I missed out on for my snow tires, and also that I missed a last-minute chance to get into a vision therapy program that I'm on a 3 month waiting list for.
Maybe as a backup/workaround until resolution, check your voicemail a couple times a day whether you have notifications or not.