Old outgoing vmail to deceased friends phone forwarded to my phone??

[merrydizzle]

Okay, so I'm sure some of you will read this and think I am crazy, but I promise this actually happened and I need answers because to be honest I am pretty freaked out. This morning I heard the voicemail notification noise, which I found odd because my phone didn't ring. When I checked it said I had a message from an "unkown number". I listened to it and was shocked to find that it was a voicemail that I had left my friend on her birthday.  I would have just shrugged it off as she accidentally somehow forwarded it to me, however, that friend has been deceased for a little over a year now.  I would assume because of this her phone would have been deactivated. Even if it weren't deactivated, I think you need to actively save your voicemails every once in a while or else they will be deleted. So my main question is, how did I get this voicemail? Where did it come from? Is there some sort of glitch that can cause this? I am utterly confused and freaked out.