Switchboard and reception operators don't tend to have access to who is in the office and who is not, other than knowing that a person is forwarded to voicemail.
At a busy switch, many operators can't wait to get rid of a caller because they would have up to 5 other calls waiting. By dispersing it to the voicemail system or admin staff then the other calls are responded to. Some companies use switchboard analysis software to see how calls are dealt with, how long people are left waiting etc.
As a rule of thumb, the best companies I have worked for operate a 3 ring policy for picking up calls. Voicemails changed everyday and being date specific, returned calls by COB and a friendly voice on the switch.
The worst use automated systems that tie you up in knots involving multiple choices and staff that don't know how to use the telephone systems!