I am sure there is a setting that addresses the problem you have, but I have been unable to either remember it or find it. I am sorry to have to confess failure on this matter, and I offer my apologies.
The best that I can offer at this point is to
1. change procedure just a trifle, and
2. change the settings to OHCA - On-Hook Call Announce.
So the procedure for the receptionist getting outside calls coming in and routing them to extensions would be
- use OHCA ... that is ring the extension, and when 'busy' is heard, press 1, and speak to announce the call through their phone's speaker (not the intercom, not all phones), and this might change the way the voicemail system picks up on such calls.
In short, I suspect that maybe the receptionist's way of trying to transfer the calls may affect whether the voicemail at the extension picks up on it.