Email to text gateways can be a little finicky.
I just did some testing... I had success using the AT&T MMS gateway @mms.att.net. Verizon's is @vzwpix.com.
You'll need to use the MMS gateway since the voicemail box is trying to send an attachment.
Are you using an Exchange server? I found that creating a rule in Outlook also worked, and this way the user can get the message in Outlook and on their phone.
The from address is specified on the e-Mail tab of the mailbox. That's where no_user@voicemail is coming from on your system. I set this to a real looking, but fake address ([email protected]
) on my system. See here for more info.