Call Forwarding concept explanation please

I’ve seen many sample dialplans for call forwarding. I’m curious tho on how this works. First how is it supposed to work when call forwarding is activated via a dialplan, and second, how is it supposed to work when activated at the phone itself? If activated via dialplan, then Asterisk receives the incoming call and initiates a second call out to the forwarded number right? So in effect there are two calls to handle one incoming call.

I had a client access a Aastra 9133i phone via the web gui and turned on call forwarding in the phone. How is this process supposed to work? When Asterisk sends an incoming call to the phone, the phone then tries to dial back out to the forwarded number?

Maybe I"m still looking at it from the old school viewpoint but when I turn on call forwarding on my land-line or voip provider, then phone call never really comes into my office, they route it to the forward number. I’m just trying to figure out the concept in Asterisk…

If you forward on the telephone, then in response to a SIP invite (for SIP phone) the phone sends back a SIP 302 Moved Temporarily (a form of redirection). Asterisk sees that along with the number where it should be making the call and acts appropriately (calls that number and bridges the call, which could involve signalling another redirection depending on the dial options).

If you do it in the dialplan than it may make another outbound call and bridge them, or it may issue another redirection depending on the dial options such that it does not have to stay in the media stream.