All visual voicemails before May 2017 suddenly deleted sometime in the last few days

This happened to me when moving from an iPhone 6 to a 6s!!! Lost some valuable voicemails. What I learned: 2 types of saved voice mail: recent saved on AT&T servers, and older ones are stored on the iPhone, and backup. The older ones are the ones that disappear. The best way to get them back after an upgrade is use third party software to identify and save them from the backup. ( Apple & AT&T will not tell you this, unless you are in person and it will be off the record ) Best thing: save them right away somewhere. Many ways to do this: email, airdrop, texting etc. etc. the problem: you have to rename them, which could take awhile if you have a lot. Representatives from AT&T, Apple, the Genius Bar people, all have little information on this problem. Apple mostly just tells you how to save them using texting etc. Neither company seems to take full responsibility, they are your voicemails saved on your phone !! they shouldn't be deleted so randomly!! (we need Steve Jobs back!! )