Lost voice mail in restore from back up?

Hello. So i just upgraded from a 4s to a 6s and i backed up my 4s before i made the switch. I just noticed that a voice mail from my late dad, that i had saved, is gone😟 I must have lost it in the restore from back up process. Im super bummed. Did i do something wrong or do you lose voice messages when you restore from a back up? I hope, i really hope, i didn't cause this by not doing something in particular. Any help would be great. Thanks. O