Since the phones are in fact working,it sounds like you have a corrupted database.

FEAT 99 won't work if you don't have a valid key pattern assigned, especially if the phone in question is configured as an attendant.

The 'INVALID' comes up if you dial an extension number that isn't currently in the database.

Do a fresh download of EVERYTHING under a new file name and start looking at things to see what's correct and what isn't.

Sometimes the thoughts in my head get so bored, they go for a stroll through my mouth. This is rarely a good thing.