The 78XX phones are relatively new and would not be supported on UC540 latest available release to my knowledge, and they would definitely not be supported with IOS 15.1.4 as that is much older than 78XX phones.
Your options would be to migrate to BE6K, or CME/CUE, or replace the phones with supported models, i.e. 79XX series.
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...