Conditional call forwarding

Some carriers call that "No Answer/Busy Transfer". So, if searching for your carrier's instructions for conditional call forwarding doesn't work, try that.

Here are the instructions my carrier, Verizon, provides:

Forward Calls Only When Busy or on Another Line

What if you want to forward calls only when you’re unable to pick up or on another call? Then the No Answer/Busy Transfer feature (also known as Conditional Call Forwarding) may be the solution. Use this feature to send calls to another phone when your phone is busy or when you don’t answer after three or four rings: Dial *71 and the number you want to forward calls to. You may activate Conditional Call Forwarding through My Verizon as well. While this feature is active, your phone will ring several times when you get an incoming call, giving you the chance to answer. If you’re on the phone or choose not to answer, the call will be forwarded to the destination phone number. You can still make outbound calls with this feature activated. To deactivate the No Answer/Busy Transfer feature, just dial *73. 

It's possible they may work for you.

Call Forwarding is a Carrier Specific feature please contact your Carrier to understand this feature better.