[Asterisk-Users] divert if not here

[Altus Syman]

Good day all
We have a pbx system running sip and sipphone(Bughtone)
My question is.If a user is not at their desk,how do I tell it if a call 
comes in it should direct it to someone else
Do I need a different phone for this?The only other way is that they 
have to switch it off and in my dialplan on stem 2 I will have to say go 
to that user?
Please give advice on this
Thaks
Altus