You should tell your old school friend that their voicemail is full. At least with my carrier, I can only have like ~20 voicemails sitting around. I listen to every voicemail I get but my voicemail inbox filled up and I didn't realize it was full for years (there was no notification or anything). To make space I had to actively go delete voicemails. Of course, deleting them just sent them to a "deleted" folder, which I had to go clear as well.