gssieg,
The issue is with the users phone as they probably set the Caller-ID to go directly to VM for some reason and haven't removed it. So, when they get a call from the number the MiVO-250 is sending out to them it goes to VM.
The simplest answers is sometimes the correct one.
Since it works with your cell phone and only one person is having the problem it isn't an issue with the phone system.
Thanks,
TE