I have made the following setup,
on the multi tenent edition i made extension 10
in the user panel i have set,
Call Forwarding on Busy enabled to extension 11,
Ring my phone and if it is not answered in 10 secs to extension 12
when i call extension 10 while it is busy extension 11 rings for 10 seconds, after that it goes to the voicemail of extension 10.
while what the costumer wishes is that after 10 seconds phone 12 would be ringing and maybe even with extension 11.