Can't leave voicemail with Poly VVX 250

[AHNCC]

I have the phone assigned.  I can make outgoing calls.  I recorded an outgoing message.  When I call the phone, it rings, and I can answer it.  However, if I don't answer, it does not go to voicemail, and the caller gets "call failed".   I have both user and admin guides, but I don't speak admin.  We only have one phone, with wifi, and a Pro account, so can't talk to anyone.  It took hours just to get this far.  Can anyone help?

[AHNCC]

With help from Zoom support, I got it working by un-assigning and re-assigning.  Now I'm trying to find out how to access remotely.  User manual says ask the Admin.  Admin manual in unintelligible.  Zoom support gave me a clue, but I have not been able to test it yet since I don't "own" the number.