I have a ring group with 2 extensions. One of them is my Nokia. The 'destination if not available' is set to voicemail. When I was testing this by not picking up any of of the extensions, I noticed that sometimes the Nokia creates a ringtone that's going on forever. I mean, the Nokia stops ringing after the specified time-out, but the party that initiates the call keeps hearing the ringtone.
But the strange thing is that seconds later I receive a voicemail in my email. It seems that allthough the Nokia extension keeps on creating a ringtone, the system has forwarded the call to the mailbox as it should. I have tested this and sure that's the case. While the calling party is hearing the ringtone he can also talk in his device and this will be recorded in the voicemail. So it looks like the ringing signal is not routed to voicemail but the voice channel is routed as specified?