Make sure once you put in the voicemail group you then press the right soft key under the display and change the currently set option to yes by pressing + on the volume.

The default no answer delay is 15 seconds unless someone has changed it.

This will only set forwarding for direct calls to this station such as DDIs and internal calls. If the DDI rings a group then you will need to setup a group overflow to voicemail and setup a key to access it.