I can also confirm that my A8 phone unit went bad but left the MOST Bus working. Only noticed when the phone icon wasn't on the bottom of my MMI screen.
When I checked, I had the same symptoms as the OP; 'Telephone Not installed' messages on the MMI screen and 'Cannot be reached' messages in VCDS for Address 77.
Lifted the carpet and no signs of water ingress; opened the ECU case but no visible issues. But had to fix it by replacing the unit with a cheap eBay replacement.
Julian
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Audi A8 3.0TDi MY2016 in Nautilus Silver.
Audi A8 3.0TDI MY2005 in Midnight Blue. Gone since 2018, after 10 years!
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