I think you'll find your definition of hacking is not what you think it is
The Free Dictionary states: To gain access to (a computer file or network) illegally or without authorization A message stored in a voicemail system is a file stored on a computer somewhere and they gained access without permission.
So what's your definition? Is it because they used such an easy to use password it's not hacking? Does that then mean brute force or the use of a dictionary or simply trying the most common passwords is not hacking? Or is it because it was a phone connecting to the network that makes it no longer hacking? Or maybe it's that they managed to gain access to her phone records without actually having possession of her phone that stops it from being hacking?