Voicemails are being deleted

I was able to hear a voicemail from my sister but after I heard it last week, it is no longer there. All of my voicemails are disappearing. I had some very sentimental voicemails and they’re gone.

also, I was overcharged by a compounding pharmacy and when I went into show them the voicemail that I got that the medicine was ready it was gone. If I could’ve produced that voicemail I wouldn’t have been charged the $20 that I was charged.

I wanna know why this is happening