This will probably be obvious to some, but it had me scratching my head for a while until I saw a post in another forum that indirectly gave me the answer.
If you are trying to use the Call Forward on Busy feature and it does not seem to be working, make sure that you do not have Call Waiting disabled at your provider (or at your switch, if you are connecting to something like an Asterisk or FreeSWITCH server). The reason is that if you have Call Waiting disabled at the provider/switch, it will never even attempt to put a second call through to the Obihai device. The switch sees that you already have one channel open, and since Call Waiting is disabled it won't even try to put any additional calls through. Since the Obihai device never receives those calls, it cannot forward them.
If possible, you should do things like call forwarding and call waiting at the provider or switch, not at the Obihai device itself. But if you have some reason for wanting to do it on the Obihai device (such as forwarding calls from one service provider account to another), then make sure that Call Waiting is off at the provider or switch. If you do not wish to enable Call Waiting, which is usually the case when you are enabling Call Forwarding on Busy, then turn it off at the Obihai device itself (I am actually not sure which feature takes precedence if both Call Waiting and Call Forwarding on Busy are set, so disabling Call Waiting may or may not be necessary).
Note that what I have said above does not apply if you have an additional incoming call from a different service provider. So if you had an incoming call on SP1 and then while you were on that call, a call came in on SP2, it would reach the Obihai device because the SP2 provider would have no knowledge of the Call Waiting setting at the provider associated with SP1. But, a second call on SP1 would still be blocked if Call Waiting were enabled at the SP1 provider. So if the Call Forwarding (on Busy) feature works sometimes but not all the time, check the Call Waiting setting at all of your service providers!
