If you get a MiCollab console, you can have your receptionist control incoming calls in this situation - you just need to buy a licence and apply it to that person's MiCollab Client profile in the MiCollab Client Server.
For voicemails, the best thing would be to use the email system to send them to the different user.
This is all a bit of a labour-intensive way to deal with people who are off sick though. It would be better if you got the console. Also if you have vital outward-facing numbers like this, best to not assign them to people, but assign them to a ring group instead, then you can pop people into that ring group when this kind of need arises.