Recovering old voicemails

I have used an iPhone (several models) over the years, but have always been with AT&T.

Each time I get a new phone, I just asked the AT&T or Apple representative I was working with to transfer my data - and I assume that included my voicemails.

I now want to retrieve all of the voicemails from a relative who's just passed away. I discover that I only have those saved back to 2017 on my current phone. Is there any possibility of getting this person's older voicemails from AT&T? Were those older voicemails even saved on an AT&T server? If Yes to both answers, how would I go about doing that?

Thank you very much for any helpful advice.