This is bad news since most voice mail systems delete after some number of days. Since they are not on your phone, only those folk that had them can answer.
My bet is they are gone.
I recently purchased an iphone x to replace my iphone 7. The iphone 7 is now being used as a backup phone with a different number and sim card. The original sim card is in the new iphone x.
I had a number of very important, irreplaceable voicemails on the iphone 7 that did not transfer with the sim card to the iphone x.
Apple tells me that they cant help me but that AT&T has them on their server. I called AT&T and they told me to switch the sim card back to the old phone and the voicemails would reappear. They did not.....!
How can I retrieve the missing voicemails? Several of them were over a year old and had been left by my father and father-in-law before each of them passed away last year. Neither Apple nor AT&T seems able or willing to help me and they have to be somewhere because I didn't delete them!!