I'm not sure why you made "number for sending calls to voicemail" field mandatory when "Enable voicemail" checkbox is on.
What I want (and what I used to do with X-Lite) is to have voicemail support so BFO check for voicemail, displays an icon when there is one, lets me check it (so I fill the "Number to dial for checking voicemail"). That is it. I do not need BFO to send caller to voicemail because my PBX does that in case I did not answer.
That is why the "Send calls to voicemail if unanswered for..." checkbox is OFF in my configuration. Why should I fill "number for sending calls to voicemail" field??? If I leave it blank, BFO says "You must enter a number to dial for checking voicemail and number to dial for sending calss to voicemail".