Blind call forwarding—All INVITE requests sent to this phone number will be redirected immediately to the phone number stored in the user_preferences table. The SIP router will fork the call creating a new leg to the new destination. The phone with call forward configured won't even ring, registered or not.

Forward on busy—In this case, we will use the failure_route feature to intercept the "486 Busy" message and create a new leg sending the INVITE request to the final destination.

Forward on no answer—If a phone replies to an INVITE request with a "408 Request Timeout", OpenSER again will use the failure_route feature to intercept the message and create a new leg sending the INVITE to the final destination.