I'm feeling a bit thwarted on all fronts today trying to do something that I thought would be a piece of cake.

Here's my current situation:
Currently have a 2degrees via fibre and using a Fritzbox 7490.  We have two phone lines setup in the fritzbox. One of them is used as an office phone number for our business and when we don't answer, it goes to 2Degrees voicemail which then gets delivered to our team via email and everyone can pickup messages left via their email accounts.

Works great

Here's what I want to change and where it goes wrong:
If I'm working at one of our other offices, I would like to to send incoming calls from my office number at home to another office number via the Fritzbox's Call Diversion feature.   Set it up and that works just fine.  

HOWEVER, the voicemail is totally unreliable now.  

When the fritzbox diverts the call to the other number and no one at that location answers after 20 seconds the call stops ringing and it diverts to 2degrees voicemail and sometimes  2degrees voicemail will pickup, play the greeting and let me leave a voicemail.  However, probably 2/3rds of the time 2degrees appears to pickup, but it doesn't play any sort of voicemail greeting.  Nothing....just dead air.  I even waited on the line for 10 minutes and nothing happened. It's totally inconsistent and I can't seem to predict what is happening. I disabled voicemail on the destination number so it definitely is not another voicemail intercepting it.

So I tried plan B:
I setup the answering machine on the fritzbox to pickup after 10 seconds.  Tested, it let me leave a message and then it forwarded the message via email.

Sweet, right?

Then I turned on Call Diversion and the Fritzbox does it's job of diverting the call, but if no one answers at the destination, the Fritzbox answering machine never engages.  As best I can tell there is no documentation or anything inside the Fritzbox 7490 that suggests that it cannot do this, but it refuses to engage the voicemail if call diversion is active. So maybe it's a firmware bug?

So that's where I'm at - either option would be fine for me if they worked.  I just cannot understand why 2degrees voicemail intercept is only working sometimes???

Since 2degrees voicemail is supposed to happen in their network I would think that it wouldn't even care what the Fritzbox is setup to do because it should intercept unanswered calls after the 20 second 'no answer' period.