I reviewed the problem with the technician that you worked with earlier today and it looks like there were a couple of issues. Any time where there was a call to the number and the call went to VM we noticed that your end point wasn't responding to the invite. This in turn invokes the unavailable call forwarding which sends the call to voicemail.
The issues that you worked on today regarding your registration brought something to light in that your firewall was re shaping traffic in a manner that was causing the service to fail.
We hope to hear from you soon once you get the firewall/router issues sorted out.