Voicemails just now coming in from a year ago

I bought an IPhone XR about two months ago. Before that, I had an Iphone 6plus for a few years. The entire time I had the 6, I never received any voicemails but didn't think anything of it. Just now after a few months of having my new IPhone, I've been getting voicemails that were left over a year ago, but they're coming in randomly (the other week I had three come in from a year ago, today I had five come in from a year ago). A lot of these were really important Voicemails too that I never responded to because I didn't even know I had them. It's making me think half the time when I get missed calls, they're leaving me voicemails but I don't get them for another year... why is this happening and what can I do to fix this?