The number for your voicemail varies depending on when the voicemail service was added to your account and also on your location.
To have the retrieval number specific to your account, you can contact customer service and an agent will be able to assist.
You may also reach your voicemail from a phone other than your Fido by calling your cell phone number and pressing on # once you have reached the voicemail greeting.
I hope this helps
One can also try this thread: