Call forwarding on VoIP.ms isn't like the hard-and-fast call forwarding you might be used to on a conventional landline or cellular phone. Instead, like all things in VoIP.ms, Call Forwarding is nothing more than the endpoint of a routing. Normally, the endpoint would be RING GROUP, in which certain phones on your account would ring and you could answer them. Unless directed to another routing object, all incoming calls following the routing specified in the Routing, Busy, No Answer, or Unreachable parameters defined within the DID.
So, say you wanted all unanswered calls on your DID to be forwarded to another number. You'd start by creating a CALL FORWARDING object that defines the destination of the call forward. You'd then go to your DID definition and change the UNANSWERED parameter to point to this newly-created CALL FORWARDING object. Now when a call goes unanswered, VoIP.ms would pass the call along to the object specified in the DID, which is now your CALL FORWARD.
How a call ends up at its defined endpoint can be as simple as this, or a complex chain of events, each providing a divergent destination, depending upon the logic you wish to follow. For instance, a DID could route incoming calls to a TIME CONDITION object, which would then choose a further object based on the result of the conditions you provided.
FILTERS can also be created that override the default routing in the DID, so even if you've set the default routing to a RING GROUP, you could create filters for certain numbers (or groups of numbers) that send the call to a different destination, such as your CALL FORWARD.
So essentially, there is no such thing as "activating" or "deactivating" a call forward. It is always around, but only does something if a routing path ends up there.
I hoped this adequately answers your question.