"Your Message has been erased/deleted. If you ..." What causes this?

[AC Question]

I I have a Droid Turbo and approx. 6 minutes of saved voicemails at all times. Every once in a while when a friend calls, they get interrupted when leaving a voicemail with: "Your message has been erased/deleted. If you would like to leave another message, wait for the beep." What might be causing this? They're calling from an AT&T iPhone but I don't believe that should matter. Thanks.