I tried Call Forward on Busy a long time ago with GV and it didn't work. Other people reported the same. I believe it works with other providers. Unconditional Call Forward worked with GV. I don't know why Busy doesn't work.
You could fork the call to your phone and to the other number, but put a delay on the call to the other number. This would give you time to answer the call before the other number starts to ring. There would always be a delay to the other number, even when the phone is busy. You would have to play with the delay for best results. This is the best I can do.
Assuming GV is defined on SP1 and you want to route the call out SP2 after 10 seconds:
Voice Services -> SP1 Services -> X_InboundCallRoute:
ph,sp2(18005551212;d=10)