The first step when setting up a call forward on SIP Manager is to create a divert rule
Select Call Forwards and then select Create
Next in the left column, select Numbers and then Manage
Select Details on the number you wish to divert (Please note, if the number does not appear in this window the divert would need to be diverted by the supplier)
Select Routing Voice, Select Call Forwards from the first drop-down list and then select your rule in the second drop-down list. To save the divert, hit select.
To remove the divert, follow the above steps from the Numbers tab and revert the routing to the desired destination.