It is just a minor configuration issue, also it is sometime a misunderstanding.
FOP2 cannot and will not alter your dialplan in any way. That means that if you do not have configured in your dialplan a special extension to go directly to voicemail, then you cannot configure FOP2 to do such thing.
To setup the voicemail extension for a button, in the button config itself you have to set something like this:
extenvoicemail=*100@from-internal
That means that for that particular button, to reach comedian mail directly you must dial *100 on the from-internal context.
So, to troubleshot, first run this command on your server to find out what voicemail extension is being configured:
/usr/local/fop2/autoconfig-buttons.sh
Inspect the output, look for the extenvoicemail for some of your extensions. What do you see there? Then try, from *your phone*, to dial that extension you see. Does it go to voicemail?
Maybe it is a bug in the FOP2 manager that is not setting the correct voicemail extension (something you can change in the buttons page itself), or perhaps it is just not configured in your system..