My issue is resolved! I left the country on vacation and upon my return my iPhone no longer receives SMS notification of voicemail messages. I now only get the iPhone's own missed call and voicemail alerts.
This change happened without any recent involvement from me. I had not yet written my letter to Rogers and my last call to them was probably in early November. I did, however, write a letter to Apple in November asking if the iPhone was at fault or if Rogers/Fido was incorrectly handling voicemail notifications. I never got a reply, though. In any case, I am a happy camper today.
I hope the rest of you have similar success stories.