I manually transferred all the WAV files for user greetings from a FreePBX 13 to 15 after doing a bulk import of extensions. See this thread:
After doing this, several people said that they could not transfer voicemails to certain extensions. They would hit transfer and it would immediately hang up. As seen in another thread I solved this for one extension by deleting and recreating the extension – but I didn’t want to manually re-create all extensions.
This appears to have been solved by re-recording their name (#3) .
So far, it appears to be just the name is needed.