Recover voicemails??

I backed up iPhone and now some of my voicemails are gone. Is there ANY way of retrieving them? They weren't deleted, therefore, are not in the delete section where I can undelete. They are from December - February. The only ones still there are from March 25 to present. My father passed away one month ago and I had voicemails from him that I had saved from December and January!! I backed up my phone to computer, but don't understand why that is all that is now missing! Please help.