Context (we're UK based): My wife had an iPhone using the SIM card of one network provider. She then switched networks (same iPhone, new SIM card). During the porting of her number to the new network, she must have received a voicemail. Once the new network had ported her number, checking the voicemail inbox of the new network yielded zero voicemails. The notification on the Phone app still said (1) voicemail (the new network doesn't support Visual Voicemail, so it simply says "Call Voicemail" in the UI). She receives a SMS every week or so to alert her to this phantom voicemail. And toggling the iPhone's Airplane mode yields the same immediate voicemail notification.
Phone the old network? They can't help her because she's no longer a customer.
Phone the new network? They can't help because her voicemail inbox is empty.
Get a new iPhone? She recently upgraded iPhone's, popped the old SIM card into her new phone and instantly received a notification for the phantom voicemail.
Leave a new voicemail and delete that one, hoping it'll clear all voicemail notifications? Yep, no difference.
Turn Phone badge notifications off, restart the iPhone, then turn Phone notifications back on? Tried it, no difference.
Switch SIM cards into a different iPhone? We swapped SIM cards into each other's iPhones. I received no voicemail notification on her iPhone, but she received a notification as soon as she put her SIM card into my iPhone.
So the voicemail seems to live with the number itself...somehow.
Now, my question is: Is the only course of action to change numbers entirely and just order a new SIM? Has anyone faced this exact issue and resolved it somehow?