Hi Faizal,
Callback won't work if this "busy" phone is set with a call Forward busy setting like
the example below;
ephone-dn 5 dual-line
number 204
label 204
description Danish Jabber
name Danish Jabber
call-forward busy 502
call-forward noan 502 timeout 14
corlist incoming MOBILE-CALLS
huntstop channel
Cheers!
Rob