True, you don't understand. I will try to explain it.
At the first, phone doesn't do call forward by self. Phone can be configured to requests call forward from PBX/Call Manager it's connected to. If configured, phone requests it for any incoming call. No, phone can't be fine-tuned to fire such request for some calls only. So does it mean you can't forward just some (not all) calls ? Definitely no. Remember - phone doesn't forward calls at all. PBX is forwarding them. Yes, PBX can forward call because of call-by-call request fired by phone. But it's not most common case. You can just turn on forwarding at PBX level. Once active, no call-by-call influence from phone's side is necessary to forward particular call. Moreover, this kind of forwarding works even in the case the phone is unreachable. And good news for you - this kind of forward is tied to particular extension. E.g. you can request forwarding for calls to specific extension while calls to other extensions will be still delivered to original phone.
Hope you understand how it works now.
Well, but how to turn on forwarding for particular extension at PBX level ? Supplemental services, like this one, are turned on/off by dialing dedicated supplemental service codes. Unfortunately, exact value of service code is PBX-specific. You need to ask PBX administrator what code needs to be dialed to configure forwarding on extension.
For better understanding I will provide example here, but it may not work for you as is - SS codes in examples may not fit your environment.
On our PBX, call forward (unconditional) to target number 123456 can be turned on by dialing *21*123456#. Once dialed from particular extension, all incoming calls for such extension will be forwarded to specified target number (calls to other extensions remain unaffected). Call forward (unconditional) can be turned off by dialing #21#
Remember - you need to ask PBX administrator for SS codes you need to use in your particular environment.
