There was a recent thread that shows a solution:
However, IMO it’s not a good idea for your use case.
What problem do you see using two phone lines? Why do you even want a cellular voice call, rather than using a SIP app on the mobile?
Also, note that using call forward this way is error prone: users forget to turn on forwarding when they leave the office or are temporarily away from their desk, and forget to turn it off when they return (until their mobile unexpectedly starts ringing). A better solution is Follow Me, always active, that starts ringing the mobile if the desk phone is not answered quickly.