If you either plan to or already is running your mailbox from Office365 – then please note that the VoicemailOnBusy actiontype will not work (outofthebox at least) – Because (strange as i may seem) the BusyOptions feature set was designed for Onprem only – and therefore hosted Voicemail is not supported – and wont work.
See if you have it enabled by running get-csvoicepolicy
The Basic BusyOnBusy function will still work great, only Voicemailonbusy as actiontype is not supported.
IF you ARE running 365 UM and still want busyonbusy with choice of Voicemail – you would have to run to 3.rd party solution like ex Routingagent from Competella.
BytheWay here is an MS article on how to activate the feature:
Or turn to my blog post on the subject:
The “Feature” of missing 365 UM support is documented here:
As always Happy SKYPE’ing