Call Forwarding

How to forward a call from one extension to other one?

Example:

call from 100 to 101
if 101 one did not answerd then transfer to 102.

how i can do that ?

You are confusing extensions and devices.

exten => 101,n,Dial(technology/address-of-first-choice-device-for-extension-101)
exten => 101,n,Dial(technology/address-of-fallback-device-for extension-101)

You may want to test DIALSTATUS and/or HANGUPCAUSE, to avoid restrict the cases in which you try the altenative.