This is certainly an odd problem. May I ask how often has this happened. What I would like to do is confirm that your voicemail provisioning is correct. To do so, may I have you send me a private message with your mobile number or account number. If needed, I will contact you to do some additional troubleshooting.
I'm having the same problem on 2 iphones. none at all or sometimes days later
i just left a message from the house phone & got the notification a few 2nds later.
working fine for me as well....