I looked back and didn't see this discussed, sorry if it's already there.
I have several DIDs and for each I've created a Voicemail (just the default "the person at extension [DID #] ..."). There may be a better way to manage this, but that's a separate discussion. Each DID has the corresponding voicemail set as "Voicemail associated with DID" in the
voip.ms website.
If I open a DID in the app and make any change to it, the voicemail assigned (4th field, under POP) also changes. It appears to be changing to the last voicemail in the list, not sure if that's coincidence. Phoning the number confirms that incorrect voicemail is associated (the automated message reads the number, and if I leave a message the email says so too).
Once that's happened, the only way to make it right is through the website (very painful on my phone, not so bad on notebook), because changing the voicemail field seems to constitute a change, resulting in the same wrong voicemail being associated.
That was longer than I intended, I'll stop now but I'm happy to provide more details privately, and happy to troubleshoot.