There are two ways you can set up a call divert - which we also refer to as 'failover number' or setting up a call forward: automatic, where a number is only diverted if the trunk it is associated with loses registration; and 'unconditional', where a number is diverted regardless of the trunk registration state.
(a) select the client's 'Numbers' tab, selected the number/DDI in question and right-click it to 'Edit' it.
(b) enter a 'Failover number'.
(c) tick 'Unconditional failover' if you require the number to be diverted in all cases, whether the trunk that the number/DDI is associated with is registered or not. If you do not tick 'Unconditional failover', calls to the number/DDI will only route to the 'Failover number' in the event that the trunk becomes unregistered, hence this is an automatic failover (which is useful, for example, if internet connectivity is lost at the client / PBX site). If you select 'Unconditional' then you must deselect it when you wish to cancel the divert.
(d) click 'OK'.